Logging raw accounting packets

[Brian Candler]

>   There's no module to do this.  There are very few reasons to do this,
> IMHO.

The reason: vendors have bugs in their accounting implementations, and we
want to be able to show them the original raw packets to prove it's not our
accounting collectors which are mis-interpreting the data.

The problem with tcpdump is being able to find quickly the packets of
interest (e.g.  given a username, or given a FreeRadius
Acct-Unique-Session-Id which is an MD5 across multiple attributes). So I
want to extract the attributes of interest and index them alongside the raw
data, or offsets into the raw data.