Pull in improvements to LLProcess termination via a commit from Nat Linden here: https://bitbucket.org/rider_linden/doduo-viewer/commits/4f39500cb46e879dbb732e6547cc66f3ba39959e?at=default

This also introduces LLContinueError for exceptions which should interrupt
some part of viewer processing (e.g. the current coroutine) but should attempt
to let the viewer session proceed.

Derive all existing viewer exception classes from LLException rather than from
std::runtime_error or std::logic_error.

Use BOOST_THROW_EXCEPTION() rather than plain 'throw' to enrich the thrown
exception with source file, line number and containing function.

cleaning up build
moved most includes of windows.h to llwin32headers.h to disable min/max macros, etc
streamlined Time class and consolidated functionality in BlockTimer class
llfasttimer is no longer included via llstring.h, so had to add it manually in several places

The change from LLProcessLauncher to LLProcess introduces the possibility of a
NULL (default-constructed) LLProcessPtr. Add certain static LLProcess methods
accepting LLProcessPtr, forwarding to nonstatic method when non-NULL but doing
something reasonable with NULL. Use these methods in LLPLuginProcessParent.

Certain use cases need to know whether the WritePipe buffer has been flushed
to the pipe, or is still pending.

Previously one might get process-terminated notification but still have to
wait for the child process's final data to arrive on one or more ReadPipes.
That required complex consumer timing logic to handle incomplete pending
ReadPipe data, e.g. a partial last line with no terminating newline. New code
guarantees that by the time LLProcess sends process-terminated notification,
all pending pipe data will have been buffered in ReadPipes.
Document LLProcess::ReadPipe::getPump() notification event; add "eof" key.
Add LLProcess::ReadPipe::getline() and read() convenience methods.
Add static LLProcess::getline() and basename() convenience methods, publishing
logic already present elsewhere.
Use ReadPipe::getline() and read() in unit tests.
Add unit test for "eof" event on ReadPipe::getPump().
Add unit test verifying that final data have been buffered by termination
notification event.

Clarify wording in some of the doc comments; be a bit more explicit about some
of the parameter fields.
Make some query methods 'const'.
Change default LLProcess::ReadPipe::getLimit() value to 0: don't post any
incoming data with notification event unless caller requests it. But do post
pertinent FILESLOT in case caller reuses same listener for both stdout and
stderr.
Use more idiomatic, readable syntax for accessing LLProcess::Params data.

If caller runs (e.g.) a Python script, it's not very helpful to a human log
reader to keep seeing LLProcess instances logged as /pathname/to/python (pid).
If caller is aware, the code can at least use the script name as the desc --
or maybe even a hint as to the script's purpose.
If caller doesn't explicitly pass a desc, at least shorten to just the
basename of the executable.

This way a caller need not spin on isRunning(); we can just listen for the
requested termination event.
Post a similar event containing error message if for any reason
LLProcess::create() failed to launch the child.
Add unit tests for both cases.

If it's useful to have contains() to tell you whether incoming data contains a
particular substring, and if it's useful for contains() and peek() to accept
an offset within that data, then it's useful to allow you to get the offset of
a desired substring within that data. But of course a find() returning offset
needs something like std::string::npos for "not found"; borrow that
convention.
Support both find(const std::string&) and find(char); the latter permits a
more efficient implementation. In fact, make find(string) recognize a string
of length 1 and leverage the find(char) implementation.
Given that, reimplement contains(mumble) as shorthand for find(mumble) != npos.
Implement find() overloads using std::search() and std::find() on
boost::asio::streambuf character iterators, rather than copying to std::string
and then using string search like previous contains() implementation.
Reimplement WritePipeImpl::tick() and ReadPipeImpl::tick() to write/read
directly from/to boost::asio::streambuf data, instead of copying to/from a
temporary flat buffer.
As long as ReadPipeImpl::tick() keeps successfully filling buffers, keep
reading. Previous implementation would only handle a long child write over
successive tick() calls. Stop on read error or when we come up short.

Also add "len" key to event data on LLProcess::getPump(). If you've used
setLimit(), event["data"].length() may not reflect the length of the
accumulated data in the ReadPipe.
Add unit test with stdin/stdout handshake with child process.

Add LLProcess::FileParam to specify how to construct each child's standard
file slot, with lots of comments about features designed but not yet
implemented. The point is to design it with enough flexibility to be able to
extend to foreseeable use cases.
Add LLProcess::Params::files to collect up to 3 FileParam items. Naturally
this extends the accepted LLSD syntax as well.
Implement type="" (child inherits parent file descriptor) and "pipe" (parent
constructs anonymous pipe to pass to child).
Add LLProcess::FILESLOT enum, plus methods:
getReadPipe(FILESLOT), getOptReadPipe(FILESLOT)
getWritePipe(), getOptWritePipe()
getPipeName(FILESLOT): placeholder implementation for now
Add LLProcess::ReadPipe and WritePipe classes, as returned by get*Pipe().
WritePipe supports get_ostream() method for streaming to child stdin.
ReadPipe supports get_istream() method for reading from child stdout/stderr.
It also provides getPump() returning LLEventPump& so interested parties can
listen for arrival of new data on the aforementioned std::istream.
For "pipe" slots, instantiate appropriate *Pipe class.
ReadPipe and WritePipe classes are pure virtual bases for ReadPipeImpl and
WritePipeImpl, respectively: all implementation data are hidden in the latter
classes, visible only in llprocess.cpp. In fact each *PipeImpl class registers
itself for "mainloop" ticks, attempting nonblocking I/O to the underlying
apr_file_t on each tick. Data are buffered in a boost::asio::streambuf, which
bridges between std::[io]stream and the APR I/O calls.
Sanity-test ReadPipeImpl by using a pipe to absorb the Python "SyntaxError"
output from the successful syntax_error test, rather than alarming the user.
Add first few unit tests for validating FileParam. More tests coming!

When we reimplemented LLProcess on APR, necessitating APR's funny callback
mechanism to sense child-process status, every isRunning() or getStatus() call
called the APR poll function that calls ALL registered LLProcess callbacks. In
other words, every time any consumer called any LLProcess::isRunning() method,
all LLProcess callbacks were redundantly fired. Change that so that the single
APR poll function is called once per frame, courtesy of the "mainloop"
LLEventPump. Once per viewer frame should be well within the realtime duration
in which it's reasonable to expect child-process status to change.
In effect, this changes LLProcess's public API to introduce a dependency on
"mainloop" ticks. Add such ticks to llprocess_test.cpp as well.

Apparently something in the Linux system header chain #defines a macro Status
as 'int'. That's just Bad in C++ land. It should at the very least be a
typedef! #undefining it in llprocess.h permits the viewer to build.

Include logic to engage Linden apr_procattr_autokill_set() extension: on
Windows, magic CreateProcess() flag must be pushed down into apr_proc_create()
level. When using an APR package without that extension, present
implementation should lock (e.g.) SLVoice.exe lifespan to viewer's on Windows
XP but probably won't on Windows 7: need magic flag on CreateProcess().
Using APR child-termination callback requires us to define state (e.g.
LLProcess::RUNNING). Take the opportunity to present Status, capturing state
and (if terminated) rc or signal number; but since most of the time all caller
really wants is to log the outcome, also present status string, encapsulating
logic to examine state and describe exited-with-rc vs. killed-by-signal.
New Status logic may report clearer results in the case of a Windows child
process killed by exception.
Clarify that static LLProcess::isRunning(handle) overload is only for use when
the original LLProcess object has been destroyed: really only for unit tests.
We necessarily retain our original platform-specific implementations for just
that one method. (Nonstatic isRunning() no longer calls static method.)
Clarify log output from llprocess_test.cpp in a couple places.

On Posix, these and the corresponding getProcessID()/getProcessHandle()
accessors produce the same pid_t value; but on Windows, it's useful to
distinguish an int-like 'id' useful to human log readers versus an opaque
'handle' for passing to platform-specific API functions. So make the
distinction in a platform-independent way.

That is, we try to pass through each args entry as a separate child-process
arvg[] entry, whitespace and all.

Much as I dislike viewer log spam, seems to me starting a child process,
killing it and observing its termination are noteworthy events.
New logging makes LLExternalEditor launch message redundant; removed.

This allows callers to pass either LLSD formatted as before -- which all
callers still do -- or an actual LLProcess::Params block.

LLProcessLauncher had the somewhat fuzzy mandate of (1) accumulating
parameters with which to launch a child process and (2) sometimes tracking the
lifespan of the ensuing child process. But a valid LLProcessLauncher object
might or might not have ever been associated with an actual child process.
LLProcess specifically tracks a child process. In effect, it's a fairly thin
wrapper around a process HANDLE (on Windows) or pid_t (elsewhere), with
lifespan management thrown in. A static LLProcess::create() method launches a
new child; create() accepts an LLSD bundle with child parameters. So building
up a parameter bundle is deferred to LLSD rather than conflated with the
process management object.
Reconcile all known LLProcessLauncher consumers in the viewer code base,
notably the class unit tests.