l4_exit() works with a reasonable sched_die_sync()

Next: Killing other tasks more cleanly, and waiting on children

include/l4/generic/scheduler.h

@@ -60,6 +60,7 @@ extern struct scheduler scheduler;
 void sched_init_runqueue(struct runqueue *rq);
 void sched_init_task(struct ktcb *task, int priority);
 void sched_prepare_sleep(void);
+void sched_die_sync(void);
 void sched_suspend_sync(void);
 void sched_suspend_async(void);
 void sched_resume_sync(struct ktcb *task);