Wait on working.

Multi-threaded apps can now wait on children to destroy.
WAIT_ON is useful when a child exists with an exit code and the pager
of the child does not want to take the hassle of destorying it via an
ipc. It provides an alternative method of synchronous thread destruction,
where the child destroys itself directly rather than the parent issuing
a destroy on it explicitly.

include/l4/generic/tcb.h

@@ -98,6 +98,9 @@ struct ktcb {
 	/* Number of locks the task currently has acquired */
 	int nlocks;
 
+	/* Task exit code */
+	unsigned int exit_code;
+
 	/* Page table information */
 	struct address_space *space;