Add qmail-autoresponder, a rate-limited autoresponder for qmail.

Features:

- Limits rate of automatic responses (defaults to a maximum of one
  message every hour).
- Will not respond to nearly every type of mailing list or bulk email.
- Will not respond to bounce messages or MAILER-DAEMON.
- Bounces looping messages.
- Can insert the original subject into the response.
- Can copy original message into response.
- Can use links in the rate-limiting data directory to limit inode usage
  to a single inode.
- Can limit responses to a certain date/time range.

(schmonz)