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.