implement accepting dmarc & tls reports for other domains

to accept reports for another domain, first add that domain to the config,
leaving all options empty except DMARC/TLSRPT in which you configure a Domain.

the suggested DNS DMARC/TLSRPT records will show the email address with
configured domain. for DMARC, the dnscheck functionality will verify that the
destination domain has opted in to receiving reports.

there is a new command-line subcommand "mox dmarc checkreportaddrs" that
verifies if dmarc reporting destination addresses have opted in to received
reports.

this also changes the suggested dns records (in quickstart, and through admin
pages and cli subcommand) to take into account whether DMARC and TLSRPT is
configured, and with which localpart/domain (previously it always printed
records as if reporting was enabled for the domain). and when generating the
suggested DNS records, the dmarc.Record and tlsrpt.Record code is used, with
proper uri-escaping.

README.md

@@ -109,16 +109,14 @@ The code is heavily cross-referenced with the RFCs for readability/maintainabili
 
 ## Roadmap
 
-- Rewrite account and admin javascript to typescript
-- Prepare data storage for JMAP
 - IMAP THREAD extension
+- Prepare data storage for JMAP
 - DANE and DNSSEC
 - Sending DMARC and TLS reports (currently only receiving)
-- Accepting/processing/monitoring DMARC reports for external domains
 - Calendaring
-- OAUTH2 support, for single sign on
 - Add special IMAP mailbox ("Queue?") that contains queued but
   not-yet-delivered messages
+- OAUTH2 support, for single sign on
 - Sieve for filtering (for now see Rulesets in the account config)
 - Privilege separation, isolating parts of the application to more restricted
   sandbox (e.g. new unauthenticated connections)