Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add missing formatting strings.

Thanks to: clang

Add the "enable at reset" functionality to trigger spectral scan upon
a channel change/reset.

Add a very skeletal spectral scan control application.

The main use for this is to manually trigger spectral scan operations
(athspectral -i athX start) whilst doing debugging.