use common style in comments: backwards compatible -> backward compatible; see Wikipedia and http://itknowledgeexchange.techtarget.com/writing-for-business/backward-compatible-or-backwards-compatible/

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1490223 13f79535-47bb-0310-9956-ffa450edef68