hurd: Factorize at/non-at functions

Non-at functions can be implemented by just calling the corresponding at
function with AT_FDCWD and zero at_flags.

In the linkat case, the at behavior is different (O_NOLINK), so this introduces
__linkat_common to pass O_NOLINK as appropriate.

lstat functions can also be implemented with fstatat by adding
__fstatat64_common which takes a flags parameter in addition to the at_flags
parameter,

In the end this factorizes chmod, chown, link, lstat64, mkdir, readlink,
rename, stat64, symlink, unlink, utimes.

This also makes __lstat, __lxstat64, __stat and __xstat64 directly use
__fstatat64_common instead of __lstat64 or __stat64.