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

[Eli Zaretskii]

> Date: Sat, 03 Aug 2013 11:38:03 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 15015@debbugs.gnu.org
> 
> In any case, we cannot just remove the other alternatives; at least
> for the MSDOS build we need to keep the HAVE_MKSTEMP version.

Of course, alternatively, we could add an implementation of 'mkostemp'
in msdos.c.