[athani] Add a simple tool to list and control ANI parameters.

This is a WIP tool I'm using to figure out why ANI is weirdly busted in my
home FreeBSD AP/STA setup. Although athstats (mostly) gets the ANI statistics
correct, ANI is making the radio deaf it doesn't recover without being disabled.

It's very WIP.

Tested:

• Carambola 2, (AR9331), AP/STA mode.