refactor sending dmarc reports for upcoming implementation for sending tls reports

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.

dmarcdb/eval_test.go

@@ -276,7 +276,7 @@ func TestSendReports(t *testing.T) {
 		return <-step
 	}
 
-	sleepBetween = func() {}
+	sleepBetween = func(ctx context.Context, between time.Duration) (ok bool) { return true }
 
 	test := func(evals []Evaluation, expAggrAddrs map[string]struct{}, expErrorAddrs map[string]struct{}, optExpReport *dmarcrpt.Feedback) {
 		t.Helper()