Python 2 is approaching end of life, and we only need it to run
the mail_diff test utility.
This patch updates mail_diff to run on Python 3, which only needed minor
changes.
Our non-delivery status notifications are quite simple today, but that
makes it much more difficult to support internationalization and
cross-language reporting.
There is a standard for internationalized DSNs, RFC 6533 (which builds
on top of the structured DSNs from RFC 3464).
This patch changes our DSN messages to be based on those standards, so
it is easier for MUAs to display reports according to the users'
languages preferences.
Note we still use message/rfc822 + 8bit to transmit the message, instead
of message/global, for compatibility reasons. This seems to be more
universally compatible, but the decision might be revisited in the
future. See RFC 5335 (section 4.6 in particular).
This patch adds a test for delivery status notifications and null address
deliveries, that check that chasquid can both receive and send DSNs.
To do this, we extend the mail_diff utility to support wildcards in the
comparisons, to skip over variable parts of the messages (like dates).
