HELO and EHLO both take a mandatory parameter, which also should be used
in the Received header.
This patch tracks and enforces that parameter, and also updates the
Received header generation to use it.
https://tools.ietf.org/html/rfc5321#section-4.4
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).