wip/dovecot-antispam 1.2.90.1 NetBSD

Spam scanner needs training. We'll occasionally have false positives and false negatives. Now this is the point where the Dovecot antispam plugin comes into play. Instead of moving mail into special folders or forwarding them to special mail addresses for retraining, the plugin offers two actions for the user:

* moving mail out of the SPAM folder * moving mail into the SPAM folder

This plugin watches these actions (and additionally prohibits APPENDs to the SPAM folder, more for technical reasons than others) and tells the spam classifier that it made an error and needs to re-classify the message (as spam/not spam depending on which way it was moved).