kernel - Fix ps/thread-exit and other related ps races

kernel - Fix ps/thread-exit and other related ps races

* Adjust sysctl_kern_proc()'s kernel thread scanning code to use a marker
  instead of depending on td remaining on its proper list.  Otherwise
  blocking conditions can rip td out from under us or move it to another
  cpu, potentially resulting in a crash or livelock.  Index the scan
  backwards to avoid live-locking continuous adds to the list.

* Fix a potential race is the zombie removal code vs a ps, p->p_token was
  being released too early.

* Adjust lwkt_exit() to wait for the thread's hold count to drop to zero
  so lwkt_hold() works as advertised.