UTIL: Add utility macro cli_creds_get_gid()
    
    The KCM responder checks the owneship of the ccache based on both UID
    and GID of the peer. In order to reuse the already existing creds
    structure, let's just add a new macro that returns the GID from the
    creds structure.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>