[net80211] Add initial VHT support routines.

This is a skeleton set based on ieee80211_ht.c. It implements some IE
parsing, some basic unfinished negotiation, and channel promotion/demotion.

However, by itself it's not enough to do VHT - notably, the actual
channel promotion for STA mode at least is done in ieee80211_ht.c as
part of htinfo_update_chw(). I was .. quite amused when I found that
out.

I'm checking this in so others can see progress rather than one huge
commit when VHT is "done" (which will likely be quite a while.)