kernel - Add per-process token, adjust signal code to use it.

kernel - Add per-process token, adjust signal code to use it.

* Add proc->p_token and use it to interlock signal-related operations.

* Remove the use of proc_token in various signal paths.  Note that proc_token
  is still used in conjuction with pfind().

* Remove the use of proc_token in CURSIG*()/issignal() sequences, which
  also removes its use in the tsleep path and the syscall path.  p->p_token
  is use instead.

* Move the automatic interlock in the tsleep code to before the CURSIG code,
  fixing a rare race where a SIGCHLD could race against a parent process
  in sigsuspend().  Also acquire p->p_token here to interlock LWP_SINTR
  handling.