Use user_from_uid(3), group_from_gid(3), uid_from_user(3) and

Use user_from_uid(3), group_from_gid(3), uid_from_user(3) and
gid_from_group(3) to avoid repeatedly looking up the same user/group.
Also keep the passwd and group files open to avoid opening and
closing them all the time. OK tb@