Preserve some kinds of signatures, reenable HTDIG option.

Pull upstream change 1629 to fix Launchpad Bug #1551075
https://bugs.launchpad.net/mailman/+bug/1551075. This fix improves
preservation of OpenPGP MIME multipart signatures, by not collapsing
a multipart with a single sub-part inside multipart/signed parts.
See the Launchpad bug report for details.

EXPERIMENTAL feature: Reenable HTDIG support by rolling the patch on our
own, and laying down instructions to do so in the Makefile. Mark Sapiro
no longer maintains the patch.

Bump PORTREVISION to 3.

Note that the upstream maintainer considers another release in a few
weeks' time, and called for help with updating translations. If you
want to help, see
http://www.mail-archive.com/mailman-users%40python.org/msg68036.html