bug#28023: fix make-temp-file race on local host

[Eli Zaretskii]

> Cc: 28023@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Thu, 10 Aug 2017 15:24:13 -0700
> 
> No, your ideas all sound good. It's simpler for the generic code, if the 
> mkdir 
> replacement acts like POSIX mkdir.

So how to proceed?  Do you want to wait until I make all those changes
on master?  Or push a branch with the changes?  Something else?