Update port: mail/mimedefang to 2.31

Update to 2.31
Added IPv6 support, Better performance and scalability on
very busy mail servers, Support for Symantec CarrierScan
Server and F-Risk FPROTD virus-scanners, Documentation
fixes, etc...

PR: ports/50102
Submitted by: Andrey V. Pevnev <andrey@mgul.ac.ru>