* dbus/dbus-protocol.h: Add DBUS_ERROR_UNIX_PROCESS_ID_UNKNOWN

	* bus/dispatch.c:
	(check_get_connection_unix_user): Debug says GetProperty; but the
	method is called GetConnectionUnixUser
	(check_get_connection_unix_process_id): New function
	(bus_dispatch_test): Actually call check_get_connection_unix_user();
	also call check_get_connection_unix_process_id()

	* bus/driver.c:
	(bus_driver_handle_get_connection_unix_process_id): New function,
	handles GetConnectionUnixProcessID on the org.freedesktop.DBus
	interface

	* dbus/dbus-auth.c:
	(handle_server_data_external_mech): Set pid from the credentials
	obtained from the socket

	* dbus/dbus-connection.c:
	(dbus_connection_get_unix_process_id): New function

	* dbus/dbus-connection.h:
	Add prototype for dbus_connection_get_unix_process_id

	* dbus/dbus-transport.c:
	(_dbus_transport_get_unix_process_id): New function

	* dbus/dbus-transport.h:
	Add prototype for _dbus_transport_get_unix_process_id