related to Bug 6841: decouple choosing a Socket vs. Socket6 module from choosing an IO::Socket::{IP,INET6,INET} module, improve compatibility with old versions of these modules

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1450858 13f79535-47bb-0310-9956-ffa450edef68