fchownat: Fix test failure on OpenBSD and Cygwin 2.9 (regr. 2025-09-20). * doc/posix-functions/fchownat.texi: Document that the dereferencing bug also affects OpenBSD and Cygwin 2.9. * m4/chown.m4 (gl_FUNC_CHOWN_CTIME): New macro, extracted from gl_FUNC_CHOWN. (gl_FUNC_CHOWN): Invoke it. * m4/fchownat.m4 (gl_FUNC_FCHOWNAT_DEREF_BUG): Guess that a platform that has the chown ctime bug also has the fchownat AT_SYMLINK_NOFOLLOW bug. * modules/fchownat (Files): Add m4/chown.m4.