RESPONDER: Add a responder sbus interface to set domain state
    
    Adds a generic responder s-bus interface that all responders implement.
    The interface currently contains methods that make it possible for a sssd
    domain to be marked as active or inconsistent by a back end.
    
    In the future, this commit will be superseded by sbus signals.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>