Add the xuid and xgid keys for mapping existing users and groups to their
respective IDs.  The differences to the uid and gid keys are:

 * The IDs of existing users and groups are prefixed with "+".
   Missing users and groups are mapped to nobody, with no leading "+".

 * Numbers as user or group names are *not* mapped onto themselves;
   callers can easily avoid such calls.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>