exit() almost there.

- Implemented reasonable way to suspend task.
  - A task that has a pending suspend would be interrupted
    from its sleep via the suspender task.
  - If suspend was raised and right after, task became about to sleep,
    then scheduler wakes it up.
  - If suspend was raised when task was in user mode, then an irq suspends it.
  - Also suspends are checked at the end of a syscall so that if suspend was
    raised because of a syscall from the task, the task is suspended before it
    goes back to user mode.

  - This mechanism is very similar to signals, and it may lead as a base for
    implementing signal handling.

- Implemented common vma dropping for shadow vm object dropping and task exiting.

include/l4/generic/scheduler.h

@@ -36,8 +36,11 @@ static inline struct ktcb *current_task(void)
 #define current			current_task()
 #define need_resched		(current->ts_need_resched)
 
+
 void sched_init_task(struct ktcb *task, int priority);
 void sched_prepare_sleep(void);
+void sched_suspend_sync(void);
+void sched_suspend_async(void);
 void sched_resume_sync(struct ktcb *task);
 void sched_resume_async(struct ktcb *task);
 void scheduler_start(void);

include/l4/generic/tcb.h

@@ -148,5 +148,7 @@ extern struct id_pool *thread_id_pool;
 extern struct id_pool *space_id_pool;
 extern struct id_pool *tgroup_id_pool;
 
+void task_process_pending_flags(void);
+
 #endif /* __TCB_H__ */