Add a new function hook to struct ieee80211com which wireless drivers

Add a new function hook to struct ieee80211com which wireless drivers
can use to process, and then acknowledge or reject, incoming AUTH
requests in hostap mode.

net80211 accepts an AUTH request from any STA which fits into the node
cache. This behaviour doesn't work for devices which have a lower limit
on concurrent STAs they can serve, so such drivers need an override.

This will be used by our athn(4) USB driver soon.

ok kevlo@