NSS: Add InvalidateGroupById handler
There are some situations where, from the backend, the NSS responder
will have to be notified to invalidate a group.
In order to achieve this in a clean way, let's add the
InvalidateGroupById handler and make use of it later in this very same
Signed-off-by: Fabiano Fidêncio <email@example.com>
Reviewed-by: Jakub Hrozek <firstname.lastname@example.org>