[net80211] Initial full-offload scan support.

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

tested:

• iwm(4) - STA mode
• ath10k port (local, not in freebsd-head yet)

Reviewed by: avos
Differential Revision: https://reviews.freebsd.org/D8262