Work around bug in CIFS and vboxsf file systems.

Work around bug in CIFS and vboxsf file systems.

The bug was observed on Ubuntu operating inside a virtual machine,
editing files mounted via CIFS or vboxsf from the MS Windows 7 host.
The workaround introduces a race condition on non-buggy hosts,
but it's an unlikely race and anyway there's a nearly identical
nearby race that can't be fixed.
* fileio.c (valid_timestamp_file_system, timestamp_file_system):
New static vars.
(Fwrite_region): Test for file system time stamp bug.
(init_fileio): New function.
* lisp.h (init_fileio): Declare it.
* emacs.c (main): Call it.