Multiple updates on sleeping, vma dropping and thread suspend.

- Updated sleeping paths such that a task is atomically put into
  a runqueue and made RUNNABLE, or removed from a runqueue and made SLEEPING.
- Modified vma dropping sources to handle both copy_on_write() and exit() cases
  in a common function.
- Added the first infrastructure to have a pager to suspend a task and wait for
  suspend completion from the scheduler.

include/l4/generic/scheduler.h

@@ -37,6 +37,7 @@ static inline struct ktcb *current_task(void)
 #define need_resched		(current->ts_need_resched)
 
 void sched_init_task(struct ktcb *task, int priority);
+void sched_prepare_sleep(void);
 void sched_resume_sync(struct ktcb *task);
 void sched_resume_async(struct ktcb *task);
 void scheduler_start(void);