Document where in scan_task the scan state can change, and potentially
deal/log a warning if the scan flags change during one of those race
windows.

It's highly likely that I need to actually sit down and replace this
scan infrastructure at some point.  It has some other side effects too -
the scan task is a blocking task scheduled in the net80211 taskqueue;
so drivers that use this taskqueue have other things not run.  Eek.

If you see this printf happen then please let me know!