dc-dlvr spam/ham training system flow

An overview of the Maildir + inotify-based spam training system Eric
uses on his mail server.  This idea may be implemented for kqueue-based
systems, too.

The idea is to use inotify (via incron) to watch for new files appearing
in Maildirs.  We only want to train seen messages as ham, and old (but
not necessarily seen) messages as spam.  The overall goal of this is to
allow a user to train their filters without leaving his favorite mail
user agent.

Every message written to Maildir involves a rename, so we only
have incron watch for IN_MOVED_TO events.

The generic flow is as follows, all for a single Unix user account:

    incron -> report-spam +-> sendmail -> MTA -> dc-dlvr -> spamc -> spamd

For public-inbox, Eric uses a separate Unix account ("pi") to add a
layer of protection from fat-fingering something.  So his report-spam
script delivers to a second recipient for training, the "pi" user:
                          +-> sendmail -> MTA -> dc-dlvr

public-inbox-learn will then internally handle the "spamc -> spamd"
delivery path as well as removing the message from the git tree.

* incron - run commands based on filesystem events: http://incron.aiken.cz/

* sendmail / MTA - we use and recommend use postfix, which includes a
                   sendmail-compatible wrapper: http://www.postfix.org/

* spamc / spamd - SpamAssassin: http://spamassassin.apache.org/

* report-spam / dc-dlvr - distributed with public-inbox in the scripts/
  directory: git clone https://public-inbox.org/public-inbox.git