basing *at functions over openat when available?

[Paolo Bonzini]

It seems to me that we took a backwards approach with mingw 
implementations of the *at functions.

Mingw is perfectly able to implement both fchmod and openat via NTDLL 
functions (NtSetInformationFile and NtCreateFile/NtOpenFile).  While 
underdocumented, these functions _are_ stable and are used for example 
by Cygwin.

While looking at it, I was surprised that there is no support for 
implementing fchmodat on top of openat+fchmod.  Are there POSIX systems 
that only implement openat and possibly O_NOFOLLOW?  In this case, a 
version of at-func.c supporting those systems would be nice to have.

Paolo