Task list is introduced to the thread library.

In order to do resource recycling we need a table structure. In the search of
one, we concluded that a task list will make things easier when we start adding
COPY and NEW space handling.

This commit is for utcb recycling. Now, it does not support thread trees more
than one level depth. Thus, to be able to test it, we preferred l4thread_destroy
instead of l4thread_exit.

conts/libl4thread/include/l4thread/thread.h

@@ -13,5 +13,6 @@ int set_utcb_params(unsigned long utcb_start, unsigned long utcb_end);
 
 int l4thread_create(struct task_ids *ids, unsigned int flags,
 			int (*func)(void *), void *arg);
+void l4thread_kill(struct task_ids *ids);
 
 #endif /* __LIB_THREAD_H__ */