MFC r241731:

MFC r241731:

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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@245267 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f