Add milter-greylist, a milter for sendmail written in C that implements
grey listing. Grey listing is a spam filtering technique, which uses a behavior
trick: spammers never resend a message when they get a temporary error,
whereas real MTA do. The idea is to refuse any mail on first attempt, and
accept it after some time has elapsed.

PR: ports/67252
Submitted by: Cyril Guibourg <aragorn+ports@teaser.fr>