when logging email addresses with IDNA domain and/or special characters or utf8 in localpart, log both native utf8 form and form with escape localpart and ascii-only domain

the idea is to make it clear from the logging if non-ascii characters are used.

this is implemented by making mlog recognize if a field value that will be
logged has a LogString method. if so, that value is logged. dns.Domain,
smtp.Address, smtp.Localpart, smtp.Path now have a LogString method.

some explicit calls to String have been replaced to LogString, and some %q
formatting have been replaced with %s, because the escaped localpart would
already have double quotes, and double doublequotes aren't easy to read.

tlsrptdb/db.go

@@ -133,7 +133,7 @@ func AddReport(ctx context.Context, verifiedFromDomain dns.Domain, mailFrom stri
 			return fmt.Errorf("unknown domain")
 		}
 		if reportdom != zerodom && d != reportdom {
-			return fmt.Errorf("multiple domains in report %v and %v", reportdom, d)
+			return fmt.Errorf("multiple domains in report %s and %s", reportdom, d)
 		}
 		reportdom = d