Add per-softc locking to if_tun:

Add per-softc locking to if_tun:

- Add tun_mtx to tun_softc.  Annotate what is (and isn't) locked by it.
- Lock down tun_flags, tun_pid.
- In the output path, cache the value of tun_flags so it's consistent
  when processing a particular packet rather than re-reading the field.
- In general, use unlocked reads for debugging.
- Annotate a couple of places where additional unlocked reads may be
  possible.
- Annotate that tun_pid is used as a bug in tunopen().

if_tun is now largely MPSAFE, although questions remain about some of
the cdevsw fields and how they are synchronized.