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.

src/glue/arm/init.c

@@ -317,6 +317,7 @@ void init_pager(char *name, struct task_ids *ids)
 	/* Task's rendezvous point */
 	waitqueue_head_init(&task->wqh_send);
 	waitqueue_head_init(&task->wqh_recv);
+	waitqueue_head_init(&task->wqh_pager);
 
 	/* Global hashlist that keeps all existing tasks */
 	add_task_global(task);