kernel - Fix a few MP races against process status

kernel - Fix a few MP races against process status

* proc->p_token is required when traversing p->p_lwp_tree.

* Reorder lwp insertion on lwp_fork() so scan races see a whole lwp
  instead of a partially initialized one.

* Check for NULL p->p_pgrp and friends when only using proc_token to
  scan the process list to deal with new processes that are still in the
  middle of being fork()'d.

Reported-by: Peter Avalos <peter@theshell.com>,
	     Antonio Huete Jimenez <tuxillo@crater.dragonflybsd.org>