kernel - Make most of the fork and exit paths MPSAFE

kernel - Make most of the fork and exit paths MPSAFE

* Remove the MP lock from numerous system calls (mainly socket calls) that
  no longer need it.

* Use proc_token in a couple of places that still need work (instead of
  the MP lock).  For example, the process group (pgrp) and several places
  which call pfind() still need to use the proc_token.

* Use the per-process p->p_token in fork1(), exit1(), and lwp_exit().
  The critical portions of these paths now have significant concurrency.

* Use the per-process p->p_token when traversing p->p_children, primarily
  aiding the kern_wait() code.  So the wait*() system calls should now
  have significant concurrency.

* Change the fgetown() API to avoid certain races.

* Add M_ZERO to the struct filedesc_to_leader allocation for safety
  purposes.