kernel - Make numerous proc accesses use p->p_token instead of proc_token.

kernel - Make numerous proc accesses use p->p_token instead of proc_token.

* pfind() zpfind() now returns a referenced proc structure, callers must
  release the proc with PRELE().  Callers no longer need to hold proc_token
  for stable access.

* Enhance pgrp, adding pgrp->pg_token and pgrp->pg_refs in addition to
  pgrp->pg_lock.  The lock is used to interlock races between fork() and
  signals while the token and refs are used to control access.

* Add pfindn(), a version of pfind() which does not ref the returned proc.
  Some code still uses it (linux emulation) ---> needs work.

* Add pgref() and pgrel() to mess with the pgrp's pg_refs.  pgrel()
  automatically destroys the pgrp when the last reference goes away.

* Most process group operations now use the per-process token instead of
  proc_token, though pgfind() still needs it temporarily.

* pgfind() now returns a referenced pgrp or NULL.

* Interlock signal handling with p->p_token instead of proc_token.

* Adjust most nice/priority functions to use the per-process token.

* Add protective PHOLD()s in various places in the signal code, the
  ptrace code, and procfs.

* Change funsetown() to take the address of the sigio pointer to match
  fsetown(), add sanity assertions.

* pgrp's in tty sessions are now ref-counted.