Add uid_from_user() and gid_from_group(), derived from pax's cache.c.

Add uid_from_user() and gid_from_group(), derived from pax's cache.c.
It replaces the existing pwcache.c functions user_from_uid(3) and
group_from_gid(3) with the pax equivalents.  Adapted from NetBSD
(mycroft) changes from our own pax's cache.c.  OK guenther@