Add support for band-steering access points to net80211.

Add support for band-steering access points to net80211.

Some access points have a feature called "band steering" where they
will try to push clients from 2 GHz channels to 5 GHz channels.
If a client sends probe-requests on both 2 GHz and 5GHz channels, and
then attempts to authenticate on a 2 GHz channel, such APs will deny
authentication and hope that the client will come back on a 5 GHz channel.

So if we fail to AUTH for any reason, and if there is a different
AP with the same ESSID that we haven't tried yet, try that AP next.
Keep trying until no APs are left, and only then continue scanning.

APs with support for this feature were provided by Mischa Peters.

ok phessler@ mpi@