faccessat, etc.: support AT_FDCWD-only use

faccessat, etc.: support AT_FDCWD-only use

* lib/at-func.c: If GNULIB_SUPPORT_ONLY_AT_FDCWD, then support
this function only if its first argument is AT_FDCWD.
Emacs wants faccessat for AT_EACCESS but not for any first-arg
values other than AT_FDCWD, so it doesn't want all the openat
machinery with fchdir etc.
* modules/faccessat, modules/fchmodat, modules/fchownat (Files):
* modules/fstatat, modules/mkdirat, modules/openat (Files):
* modules/unlinkat (Files):
Remove lib/openat-priv.h, as at-internal supplies this file.
Removing this file here allows us to support programs like Emacs
that avoid at-internal.