Work around an fchownat bug in glibc-2.4:

Work around an fchownat bug in glibc-2.4:
http://lists.ubuntu.com/archives/ubuntu-users/2006-September/093218.html
This bug would cause "chown -RP ... DIR" to follow symlinks in DIR,
in spite of the -P option.
* m4/openat.m4 (gl_FUNC_FCHOWNAT, gl_FUNC_FCHOWNAT_DEREF_BUG):
New macros.
(gl_PREREQ_OPENAT): Require gl_FUNC_FCHOWNAT.
* modules/openat (Files): Add lib/fchownat.c.
* lib/openat.c (fchownat): Don't define here.  Move to...
* lib/fchownat.c: ...this new file.