SBUS: Refactor sbus_message_handler to retrieve caller ID
    
    sbus_message_handler() is refactored so that it only verifies the
    message matches any of the interfaces SSSD listens on in order to be
    able to return either of HANDLED or NOT_YET_HANDLED.
    
    If the interface is one of those the SSSD should handle, the caller ID
    is first resolved using sbus_get_sender_id_send and only then passed on
    to the appropriate special handler.
    
    If the sernder ID can't be retrieved, the message is dropped with an
    error.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Stef Walter <stefw@redhat.com>
    (cherry picked from commit dbf4bb232fdc38e741973822c64300f28f17af3a)