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.

dns/dns.go

@@ -56,6 +56,12 @@ func (d Domain) ASCIIExtra(smtputf8 bool) string {
 // Strings returns a human-readable string.
 // For IDNA names, the string contains both the unicode and ASCII name.
 func (d Domain) String() string {
+	return d.LogString()
+}
+
+// LogString returns a domain for logging.
+// For IDNA names, the string contains both the unicode and ASCII name.
+func (d Domain) LogString() string {
 	if d.Unicode == "" {
 		return d.ASCII
 	}