bug#15015: Fix some minor races in hosts lacking mkostemp

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 03 Aug 2013 00:40:05 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: Eli Zaretskii <eliz@gnu.org>
> 
> Here's a proposed patch to fix some minor races in hosts lacking mkostemp.
> I've tested this on Fedora 17 and Solaris 9.  Eli recently wrote that he
> didn't think it was feasible to close the races on Microsoft platforms,
> so I'm posting this on bug-gnu-emacs first (with a CC: to Eli to give him
> a heads-up).  It looks to me like it may close the races on Microsoft platforms,
> at least if they support O_EXCL, and at any rate I hope it doesn't hurt
> on those platforms.
> 
> Most of this patch consists of code copied automatically from Gnulib.
> The meat of the patch is in src/callproc.c and src/filelock.c.

Thanks.  However, I'm reluctant to use the gnulib mkostemp module,
because it relies on quite a few functions that are redefined in
Emacs, and I have no idea what will this do to the gnulib
implementation.  Also, since we still didn't remove the support for
MS-Windows builds that use nt/configure.bat and the makefile-w32.in
files, the uses of mktemp in the Emacs sources should be left alone
for now, if we go this route.

I thought of a far easier solution: use the same code as sys_mktemp
does, but use 'open' or '_sopen' instead of faccessat.  I think this
will solve the problem much more locally and with much smaller impact.
If you agree, I will work on providing 'mkostemp' for w32.

In any case, we cannot just remove the other alternatives; at least
for the MSDOS build we need to keep the HAVE_MKSTEMP version.