bug#13539: Use fdopendir, fstatat and readlinkat, for efficiency.

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  13539@debbugs.gnu.org
> Date: Wed, 30 Jan 2013 14:24:06 -0500
> 
> >> perhaps a further optimization would be to avoid the call to
> >> file_name_completion_stat if completion-ignored-extensions is devoid
> >> of elements that end in a slash.
> > Yes, something like that might make sense.
> 
> Maybe.  But maybe not.  The default value of
> completion-ignored-extensions includes elements that end in a slash, and
> I think fairly few uses of try-completion rebind it to something else.

I agree.  It just sounds excessive to invoke lstat/stat _twice_ just
to support this one feature.

Perhaps at least systems that have d_type could avoid one of these 2
calls.  (d_type can easily be added to the Windows emulation.)