* bus/driver.c
	(bus_driver_handle_get_connection_unix_security_context): New function.
	(message_handlers): Add.

	* bus/selinux.c (bus_selinux_append_context): New function; appends
	security context to message.

	* bus/selinux.h: Prototype.

	* dbus/dbus-protocol.h (DBUS_ERROR_UNIX_SECURITY_CONTEXT_UNKNOWN): New.