Fix some stat-related races.

Fix some stat-related races.

* fileio.c (Fwrite_region): Avoid race condition if a file is
removed or renamed by some other process immediately after Emacs
writes it but before Emacs stats it.  Do not assume that stat (or
fstat) succeeds.
* image.c (slurp_file): Resolve the file name with fopen + fstat
rather than stat + fopen.
(pbm_read_file) [0]: Remove unused code with stat race.
* process.c (allocate_pty) [HAVE_PTYS && !PTY_ITERATION && !PTY_OPEN]:
Remove ineffective code with stat race.