mod_spam_filter: monitor / block egress spam

[Georg Lukas]

The module so far can detect and block incoming spam, but for a public server deployment it's also very important to identify spam bot accounts that were registered on your own server.

Therefore, the module should also match the rules on messages sent by local users, put those users into quarantine where they can't send any more messages / subscriptions and let the admin know that there is a problem.