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.

README.md

@@ -8,7 +8,8 @@ See Quickstart below to get started.
 - SMTP (with extensions) for receiving, submitting and delivering email.
 - IMAP4 (with extensions) for giving email clients access to email.
 - Webmail for reading/sending email from the browser.
-- SPF/DKIM/DMARC for authenticating messages/delivery, also DMARC reports.
+- SPF/DKIM/DMARC for authenticating messages/delivery, also DMARC aggregate
+  reports.
 - Reputation tracking, learning (per user) host-, domain- and
   sender address-based reputation from (Non-)Junk email classification.
 - Bayesian spam filtering that learns (per user) from (Non-)Junk email.
@@ -113,7 +114,7 @@ https://nlnet.nl/project/Mox/.
 
 ## Roadmap
 
-- Sending DMARC and TLS reports (currently only receiving)
+- Sending TLS reports (currently only receiving)
 - Authentication other than HTTP-basic for webmail/webadmin/webaccount
 - Per-domain webmail and IMAP/SMTP host name (and TLS cert) and client settings
 - Make mox Go packages more easily reusable, each pulling in fewer (internal)