this also has changes to make the dmarc report sending implementation more
similar to the tls reports implementation.
- factor out code to compose a dmarc report message to the message package
(from dmarcdb for reports), it will be shared soon.
- spread emails with dmarc reports over 45 minutes (it runs hourly), with at
most 5 mins in between reports. to prevent bursts of messages. properly abort
all sending attempts at mox shutdown.
- add use of missing error details in an error path.
- fix dmarc report message subject header by adding missing <>'s around report-id.
- fix dmarc report attachment filename syntax by leaving "unique-id" out.
in smtpserver, we store dmarc evaluations (under the right conditions).
in dmarcdb, we periodically (hourly) send dmarc reports if there are
evaluations. for failed deliveries, we deliver the dsn quietly to a submailbox
of the postmaster mailbox.
this is on by default, but can be disabled in mox.conf.
