[ifconfig] add initial VHT (802.11ac) configuration and channel support to ifconfig.

This is very preliminary and mostly enough for me (with other patches)
to work on VHT support.

It adds:

• VHT20, VHT40 and VHT80 regulatory/band awareness
• VHT20, VHT40 and VHT80 channel configuration / population
• Parses vht channel specifications (eg ifconfig wlan0 create wlandev athp0 wlanmode monitor channel 36:vht/80)
• Configuration of VHT, VHT40, VHT80, VHT80+80, VHT160 channel width (IEEE80211_FVHT_VHT* flags in net80211)

TODO:

• No VHT80+80 or VHT160 channels yet - I don't yet have hardware, and I'm not yet sure how to support/populate VHT80+80 channels.
• No, I won't update the manpage until this is "more done", lest someone tries using vht and gets upset with me.
• No, I won't commit the regulatory database I'm testing with, so you'll just end up with no VHT channels ever populated. Which is good, as there isn't an 11ac driver in-tree yet to try it with.