net80211: improve scan debugging

While debugging other problems I ran into the case where net80211
was thinking a scan was ongoing and new scans could not be started
but given other logging there was clearly no more scan running.
It was hard after the fact to quickly determine enough state to
reconstruct or validate assumptions.

Improve a MSG_SCAN debug logging and implement _db_show_scan() ddb
output which can be printed along with show com /S or /a.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 33396ec29f1daae31f4738f0da98dce168ca73c6)