kernel - Fix exit races which can lead to a corrupt p_children list

kernel - Fix exit races which can lead to a corrupt p_children list

* There are a few races when getting multiple tokens where a threaded
  process is wait*()ing for exiting children from multiple threads
  at once.

  Fix the problem by serializing the operation on a per-child basis,
  and by using PHOLD/PRELE prior to acquiring the child's p_token.
  Then re-check the conditions before accepting the child.

* There is a small chance this will also reduce or fix VM on-exit races
  in i386, as this bug could result in an already-destroyed process
  being pulled off by the racing wait*().  Maybe 25% chance.