Windows: shell extension is multithreaded

Windows: shell extension is multithreaded

Since the shell extension is multithreaded and it is possible
for more than one thread to be executing in the gui2fs.cpp module
at a time, it is not safe to use a single static 'space' buffer
by more than one thread at a time.   Move the buffer into the
stack of each function that uses it so that we have thread safety.

Change-Id: Idbec3e0027fc7e3c43b503c55c1b479bcb5984bc
Reviewed-on: http://gerrit.openafs.org/4819
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>