* bus/driver.c (bus_driver_handle_get_service_owner):
	implement a GetServiceOwner method.
	* doc/dbus-specification.xml: document it.
	* dbus/dbus-errors.h: add a 'ServiceHasNoOwner' error.

	* glib/dbus-gproxy.c (dbus_gproxy_new_for_service_owner):
	implement, using the bus GetServiceOwner method.

	* test/glib/test-dbus-glib.c:
	use dbus_gproxy_new_for_service_owner so that we can receive the
	signal.