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.

tasks/fs0/main.c

@@ -113,6 +113,9 @@ void handle_fs_requests(void)
 		ret = pager_notify_fork(sender, (l4id_t)mr[0], (l4id_t)mr[1],
 					(unsigned long)mr[2]);
 		break;
+	case L4_IPC_TAG_NOTIFY_EXIT:
+		ret = pager_notify_exit(sender, (l4id_t)mr[0]);
+		break;
 
 	default:
 		printf("%s: Unrecognised ipc tag (%d) "