Unsubscribe endpoints for mlmmj users (and possibly Mailman, too)

* examples/unsubscribe.milter filters outgoing messages
  and appends an HTTPS URL to the List-Unsubscribe header.
  This List-Unsubscribe header should point to the PSGI
  described below.  You may edit the archive_addr sub
  to disable List-Unsubscribe headers for well-known archiver
  addresses to prevent saboteurs from stopping archival.

* examples/unsubscribe.psgi is a PSGI which needs to run
  as the mlmmj user with permission to run mlmmj-unsub.
  This depends on the PublicInbox::Unsubscribe module
  which may be extracted from the rest of public-inbox.
  It is strongly recommended to NOT run the rest of the
  public-inbox WWW code in the same process as this PSGI.
  (The public-inbox WWW code will never need write
   permissions to anything besides stderr).

* Both the .milter and .psgi examples are bundled with
  systemd service and socket activation examples.
  AFAIK no other PSGI server besides public-inbox-httpd
  supports systemd socket activation.

To wire up the milter for postfix, I use the following
in /etc/postfix/main.cf:

  # Milter configuration
  milter_default_action = accept
  milter_protocol = 2

  # other milters may be chained here (e.g. opendkim)
  # chroot users will need to adjust this path
  smtpd_milters = local:/var/spool/postfix/unsubscribe/unsubscribe.sock

  # This is not needed for mlmmj since mlmmj uses SMTP:
  # non_smtpd_milters = local:/var/spool/postfix/unsubscribe/unsubscribe.sock

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License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>