Fix broken up NIS groups for compat NSS module.

The check for the inclusion of a group in the result gave up too early
in case of broken-up NIS groups.  We now fall back automatically to
the slow mode of using getgrent_r.  As an optimization, if there is
not blacklist we need not perform the check in the first place and
therefore can just accept the results of the initgroups_dyn callback.