Port to Tandem NSK/OSS, which lacks fchdir. Problem reported by

Port to Tandem NSK/OSS, which lacks fchdir.  Problem reported by
Matthew Woehlke in
<http://lists.gnu.org/archive/html/bug-gzip/2006-11/msg00013.html>.
* bootstrap.conf (gnulib_modules): Remove openat, as it pulls in
fchdir.
* configure.ac: Check for fdopendir.
* gzip.c: Don't include openat.h; no longer needed.
(treat_dir) [!HAVE_FDOPENDIR]: Fall back on the old opendir method
since fdopendir is not available.