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/mm0/src/clone.c

@@ -22,7 +22,7 @@ int vfs_notify_fork(struct tcb *child, struct tcb *parent)
 {
 	int err = 0;
 
-	printf("%s/%s\n", __TASKNAME__, __FUNCTION__);
+	// printf("%s/%s\n", __TASKNAME__, __FUNCTION__);
 
 	l4_save_ipcregs();