net80211: improve ddb output for sta and rate

Add more missing fields to the ddb output of sta (including spares so
people will notice if they use them to adjust ddb as well).
There are still more missing fields (some left as reminder in an
uncompiled notyet block).

Add _db_show_rate() to print iv_rate as well optionally including
function pointers.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days