implement outgoing dmarc aggregate reporting

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.

dmarcrpt/feedback.go

@@ -1,9 +1,14 @@
 package dmarcrpt
 
+import (
+	"encoding/xml"
+)
+
 // Initially generated by xsdgen, then modified.
 
 // Feedback is the top-level XML field returned.
 type Feedback struct {
+	XMLName         xml.Name        `xml:"feedback" json:"-"` // todo: removing the json tag triggers bug in sherpadoc, should fix.
 	Version         string          `xml:"version"`
 	ReportMetadata  ReportMetadata  `xml:"report_metadata"`
 	PolicyPublished PolicyPublished `xml:"policy_published"`
@@ -26,6 +31,9 @@ type DateRange struct {
 
 // PolicyPublished is the policy as found in DNS for the domain.
 type PolicyPublished struct {
+	// Domain is where DMARC record was found, not necessarily message From. Reports we
+	// generate use unicode names, incoming reports may have either ASCII-only or
+	// Unicode domains.
 	Domain           string      `xml:"domain"`
 	ADKIM            Alignment   `xml:"adkim,omitempty"`
 	ASPF             Alignment   `xml:"aspf,omitempty"`