Replace our version of the pwcache(3) API with NetBSD's implementation.

This adds two features:

• uid_from_user() and gid_from_group() as the reverse of user_from_uid() and groups_from_gid().
• pwcache_userdb() and pwcache_groupdb() which allow alternative lookup functions to be used. For example lookups from passwd and group databases in a non-standard location.