(dbus-name-owner-changed-handler): Replace "signal" by "member".
(dbus-check-event): Add serial number to event.
(dbus-handle-event): Apply return message if needed.
(dbus-event-serial-number): New defun.
(dbus-event-service-name, dbus-event-path-name)
(dbus-event-interface-name, dbus-event-member-name): Adapt
implementation to serial number.