kernel - Fix ptrace signal races between child and parent

kernel - Fix ptrace signal races between child and parent

* Fix several ptrace signal races which could cause the parent's wait()
  function to block forever (or until the parent is killed).  Generally
  speaking we have to use the process token to interlock the wakeup()/tsleep()
  sequence used by the ptrace code.

* Fix a MP race in the wait code, we must hold the child process's token
  when manipulating its p_flag field.

Reported-by: corecode, tuxillo