Checkpoint work in progress:
- Move PCB structures under __INPCB_PRIVATE, adjust most of the callers
  and thus make IPv4 PCB structures mostly opaque.  Any volunteers for
  merging in6pcb with inpcb (see rpaulo-netinet-merge-pcb branch)?
- Move various global vars to the modules where they belong, make them static.
- Some preliminary work for IPv4 PCB locking scheme.
- Make raw IP code mostly MP-safe.  Simplify some of it.
- Rework "fast" IP forwarding (ipflow) code to be mostly MP-safe.  It should
  run from a software interrupt, rather than hard.
- Rework tun(4) pseudo interface to be MP-safe.
- Work towards making some other interfaces more strict.
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
- Add some initial locking to the IPv4 PCB.
- Rename inpcb_lookup_*() routines to be more accurate and add comments.
- Add some comments about connection life-cycle WRT socket layer.