Convert to new uid/gid option parsing helpers

From: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/58862d35-a026-4866-ab7f-fa09dda8ac1f@redhat.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>