#4489: Use dir_fd in rmdir in _rmtree_safe_fd()
#4489: Use dir_fd in rmdir in _rmtree_safe_fd()

Now that rmdir supports dir_fd, we also use it. Attackers can't even delete
empty directories anymore.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator