Log RADIUS accounting data

[Dávid Benko]

There is a long standing (over a decade) TODO on processing RADIUS server
accounting requests:
https://w1.fi/cgit/hostap/tree/src/radius/radius_server.c?id=012a893c469157d5734f6f33953497ea6e3b0169#n1677

I created a pair of patches implementing a new callback interface for
accounting requests (patch 1) and the callback which simply logs all received
messages (patch 2). Configuration option to turn it on or off is also included.

Even though the TODO mentions writing accounting information to a file or
database, I think logging is well sufficient for many of hostapd's use cases
and definitely better than nothing.

Dávid Benko