Wrap coroutine call in try/catch in top-level coroutine wrapper function
LLCoros::toplevel(). Distinguish exception classes derived from
LLContinueError (log and continue) from all others (crash with LL_ERRS).

Enhance CRASH_ON_UNHANDLED_EXCEPTIONS() and LOG_UNHANDLED_EXCEPTIONS() macros
to accept a context string to supplement the log message. This lets us replace
many places that called boost::current_exception_diagnostic_information() with
LOG_UNHANDLED_EXCEPTIONS() instead, since the explicit calls were mostly to
log supplemental information.

Provide supplemental information (coroutine name, function parameters) for
some of the previous LOG_UNHANDLED_EXCEPTIONS() calls. This information
duplicates LL_DEBUGS() information at the top of these functions, but in a
typical log file we wouldn't see the LL_DEBUGS() message.

Eliminate a few catch (std::exception e) clauses: the information we get from
boost::current_exception_diagnostic_information() in a catch (...) clause
makes it unnecessary to distinguish.

In a few cases, add a final 'throw;' to a catch (...) clause: having logged
the local context info, propagate the exception to be caught by higher-level
try/catch.

In a couple places, couldn't resist reconciling indentation within a
particular function: tabs where the rest of the function uses tabs, spaces
where the rest of the function uses spaces.

In LLLogin::Impl::loginCoro(), eliminate some confusing comments about an
array of rewritten URIs that date back to a long-deleted implementation.

set_consuming(true) tells each postAndSuspend() call to consume the event for
which it is suspending.

Our first cut at tracking the "current" coroutine simply reset the pointer to
NULL every time we context-switched away. But that strategy doesn't handle the
case of coroutine A launching coroutine B.
Introduce LLCoros::CoroData to track, among other things, the previous value
of the current-coroutine pointer each time we switch into a coroutine. Restore
THAT value when we switch back out.

To date, the coroutine helper functions in lleventcoro.h have been in the
global namespace. Migrate them into llcoro namespace, and fix references.
Specifically, LLVoidListener => llcoro::VoidListener, and voidlistener(),
postAndWait(), both waitForEventOn(), postAndWait2(), errorException() and
errorLog() have been moved into llcoro.
Also migrate new LLCoros::get_self() and Suspending to llcoro:: namespace.
While at it, I realized that -- having converted several lleventcoro.h
functions from templates (for arbitrary 'self' parameter type) to ordinary
functions, having moved them from lleventcoro.h to lleventcoro.cpp, we can now
migrate their helpers from lleventcoro.h to lleventcoro.cpp as well. This
eliminates the need for the LLEventDetail namespace; the relevant helpers are
now in an anonymous namespace in the .cpp file: listenerNameForCoro(),
storeToLLSDPath(), WaitForEventOnHelper and wfeoh().

lleventcoro_test.cpp runs clean (as modified for new API), and all the rest
builds clean, but the resulting viewer is as yet untested.

BUG-2707 make use of OsOutputDebugString _DEBUG only on Windows to avoid throwing unhandlable exceptions in coroutines in RelWithDebInfo builds

Introduce LLCoros::setStackSize(), with a compile-time default value we hope
we never have to use. Make LLAppViewer call it with the value of the new
settings variable CoroutineStackSize as soon as we've read settings files.
(While we're at it, notify interested parties that we've read settings files.)
Give CoroutineStackSize a default value four times the previous default stack
size. Make LLCoros::launch() pass the saved stack size to each new coroutine
instance.
Re-enable lleventcoro integration test. Use LLSDMap() construct rather than
LLSD::insert(), which used to return the modified object but is now void.

In autobuild.xml, specify today's build of the Boost package that includes the
Boost.Context library, and whose boost::dcoroutines library uses Boost.Context
exclusively instead of its previous context-switching underpinnings (source of
the ucontext.h dependency).
Add BOOST_CONTEXT_LIBRARY to Boost.cmake and Copy3rdPartyLibs.cmake. Link it
with the viewer and with the lllogin.cpp test executable.
Track new Boost package convention that our (early, unofficial) Boost.Coroutine
library is now accessed as boost/dcoroutine/etc.h and boost::dcoroutines::etc.
Remove #include <boost/coroutine/coroutine.hpp> from
llviewerprecompiledheaders.h and lllogin.cpp: old rule that Boost.Coroutine
header must be #included before anything else that might use ucontext.h is
gone now that we no longer depend on ucontext.h. In fact remove
-D_XOPEN_SOURCE in 00-Common.cmake because that was inserted specifically to
work around a known problem with the ucontext.h facilities.

This fixes the Release-build crash in lllogin_test.cpp.

boost::bind() to pass any other coroutine arguments. This allows us to remove
the LLCoroBase and LLCoro constructs, directly storing a coroutine object in
our ptr_map. It also allows us to remove the multiple launch() overloads for
multiple arguments. Finally, it lets us move most launch() functionality into
a non-template method.

instances. LLCoros::launch() intends to address three issues:
- ownership of coroutine instance
- cleanup of coroutine instance when it terminates
- central place to twiddle MSVC optimizations to bypass DEV-32777 crash.
Initially coded on Mac; will address the third bullet on Windows.
Adapt listenerNameForCoro() to consult LLCoros::getName() if applicable.
Change LLLogin::Impl::connect() to use LLCoros::launch().
LLCoros::getName() relies on patch to boost::coroutines::coroutine::self to
introduce get_id().