Report/[io](packets|errors)

Hi Martin,

I've made a few gradual changes to athn(4) -- presumably getchans now
behaves as before -- and fixed (I believe) error counting for ath(4).

I'm taking

> Since there is no such single interface anymore, errors are accounted later (at the VAP level) if a VAP/node is identified for the packet. Otherwise just increment the global ic->ic_ierrors counter for errors with incoming packets that can not be associated with a concrete network (or similar ic->ic_oerrors for output errors).

to mean ic_[io]errors should be incremented only if the counter on the
VAP isn't.  It seems [io]packets is handled by net80211.

The changes I made so far to ath(4) are still all together at
https://jonathan50.github.io/gsoc-blog/ath-progress.patch

So athn(4) now works on 2.4GHz and 5GHz (but I changed some of the
code for USB athn(4) just to get it to compile, which probably ought
to be reverted, or properly converted).

Jeandre