Checkpoint work in progress:
- Initial split of the protocol user-request method into the following
  methods: pr_attach, pr_detach and pr_generic for old the pr_usrreq.
- Adjust socreate(9) and sonewconn(9) to call pr_attach without the
  socket lock held (as a preparation for the locking scheme adjustment).
- Adjust all pr_attach routines to assert that PCB is not set.
- Sprinkle various comments, document some routines and their locking.
- Remove M_PCB, replace with kmem(9).
- Fix few bugs spotted on the way.
sync with head