The trouble lines are:
			U8 * buffer = (U8 *) ALLOCATE_MEM(LLImageBase::getPrivatePool(), total_size);
			if (cur_size > 0)
			{
				memcpy(buffer, mFormattedImage->getData(), cur_size);
			}

If 'cur_size > mHttpReplyOffset + append_size' then 'total_size -= src_offset' will cause
total_size to be smaller than cur_size causing a write access violation on the memcpy.

Since the response is invalid it seemed best to make it follow the other failed partial condition.
(transplanted from 737e28ec6b4d74f3ff915a4effc13d7b615a6a9b)

Now that LLManifest.prefix() supports use as a context manager:

    with self.prefix(...):
        ...

convert existing calls to that form.

This was an interesting exercise because it surfaced at least two places where
the indentation did not match the self.prefix() nesting, plus another place
where existing code was undented without a self.end_prefix() call. (That last
was an uncaught logic bug.) This underscores the value of using a SINGLE
consistent, idiomatic mechanism to limit the scope of each self.prefix() call.

LLManifest.prefix() dates back to before Python had a 'with' statement or the
notion of a context manager. That's why every prefix() call requires a
corresponding end_prefix() call.

Existing usage is of the form:

    if self.prefix(...some args...):
        self.path(...)
        ...
        self.end_prefix()

The use of an 'if' statement is solely to allow the coder to indent the
statements between the self.prefix() call and the corresponding call to
self.end_prefix() -- there is no intention to make that code conditional.
self.prefix() unconditionally returned True to facilitate that usage.

But now that we have the 'with' statement, this all feels a little silly. Make
prefix() return an instance of a context-manager class so that it's reasonable
to say instead:

    with self.prefix(...some args...):
        self.path(...)
        ...

and have the Right Things happen simply by leaving the 'with' block.

The only tricky part is code to preserve compatibility with old-style usage:

* The context manager has a __nonzero__() method so that if it's tested in an
  'if' statement, it can unconditionally return True.

* On leaving the 'with' block, rather than simply popping the top of each
  prefix stack, the context manager restores its length to the same length it
  had before that prefix() call. This allows for (erroneous but hardly
  unlikely) usage of the form:

    with self.prefix(...some args...):
        self.path(...)
        ...
        self.end_prefix()

Restoring the previous length makes the context manager insensitive to whether
or not end_prefix() has popped the most recent prefix() entries.

The new helper functions check_curl_easy_setopt() and
check_curl_multi_setopt() encapsulate the pervasive idiom:

    code = curl_{easy,multi}_setopt(handle, option, arg);
    check_curl_{easy,multi}_code(code, option);

But since each of these helper functions contains its own local CURL{,M}code
variable 'code', having a caller-scope variable reused for every such call is
no longer necessary -- in fact is no longer used at all. That produces a fatal
warning with MSVC. Get rid of those now-unused variables.

Mani captured that snapshot back when CMake version 2.8 was newer than what
was running on our build systems. Now we have to assume that the bundled
GetPrerequisites.cmake is better than our old snapshot. Use the bunded one.

To support that, enhance path_optional() to accept wildcards and return the
expanded filenames.

The nghttp2 autobuild package has copyright information that embeds a newline.
autobuild install --copyrights correctly produces that information onto two
lines. But that means packages-formatter.py must process any lines that do not
match its expected 'packagename: copyright' pattern as the continuation of the
preceding package's copyright information.

Since the processing for autobuild install --versions is so very similar, fold
both into the same outer loop.

Also report all duplicates for any package, instead of stopping at the first.

generalize exception catching for platform specific code

* do not redirect stderr to stdout
* catch errors generated in platform specific code and display them more nicely
* run_command no longer captures output (only used in one place;
  replaced that with direct use of subprocess)