user_from_uid(3) and uid_from_user(3) use to lookup user information.
- Implement pwcache_groupdb(3), which changes the routines that
group_from_gid(3) and gid_from_group(3) use to lookup group information.
- Ensure that private functions in pwcache.c are declared static
- Use strlcpy(3) instead of strncpy(3)