The default INFO logs are more oriented towards debugging and can be
a bit too verbose when looking for high-level information.
This patch introduces a new "maillog" package, used to log messages of
particular relevance to mail transmission at a higher level.
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).