net80211: factor out the priv(9) checks into OS specifc code.

Factor out the priv(9) checks into OS specifc code so other OSes can equally
implement them.  This sorts out those XXX in the net80211 code.
We provide 3 arguments (cmd, vap, ifp) where available to the functions, in
order to allow other OSes to use that data but also in case we'd add auditing
to these check to have the information available. For now the arguments are
marked __unused.