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/include/syscalls.h

@@ -30,4 +30,5 @@ int pager_update_stats(struct tcb *sender, unsigned long vnum,
 int pager_notify_fork(struct tcb *sender, l4id_t parid,
 		      l4id_t chid, unsigned long utcb_address);
 
+int pager_notify_exit(struct tcb *sender, l4id_t tid);
 #endif /* __FS0_SYSCALLS_H__ */