wlan - Import initial full-offload scan support from FreeBSD.

wlan - Import initial full-offload scan support from FreeBSD.

This is a very simple addition to the net80211 scan support.

It doesn't implement a replacement scan interface - it just disables
the pieces that we should disable to make this lifecycle a bit
more managable.

There's more work to come before full scan offload support is available
but it should be good enough for driver work.

* add a flag to say "full offload"
* don't do probe requests when scanning full-offload - firmware can do that
* don't do powersave transitions and buffering - firmware can do that
* don't abort a background scan upon reception of a single packet.
  Full offload drivers don't need this behaviour - they do it in firmware.

Taken-From: FreeBSD (svn r307602, r308007 and r308008)