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.
