Yet another ClamAV filter for Courier-MTA

There are some other egregious ClamAV filters for Courier-MTA out there: * Gordon Messmer's courier-pythonfilter/clamav.py * Julian Mehnle's Courier::Filter::Module::ClamAVd * Tony Di Monaco's ClamCour Those three projects started in 2003-2004, more or less in the order listed. At that time, avfilter v.1 used Sophos as a backend scanner. I thought an Open Source anti-virus package could not be workable, because of the total committment required to update virus signatures. My bad. In fact, distributing Sophos' include files had always been problematic, and then the whole interface was apparently discontinued in April 2014; avfilter v.2 is there if anyone feels like porting it. Meanwhile ClamAV, under the aegis of Cisco, is getting better and better. Eric Raymond's prophecies seem to be coming true... Besides swapping the backend engine, rewriting included a farewell to MIME parsing, a vestige of the time when scanners didn't know about email. Nowadays, scanners deal more and more with phishing, an arena disputed between anti-virus and anti-spam. Anti-phishing implies false positives. Therefore, the configuration has been enhanced so as to allow certain infected messages to pass, with an added header field spelling the name(s) of the malware detected therein. See the docs for details.