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.

main.go

@@ -1442,7 +1442,7 @@ can be found in message headers.
 			} else {
 				spfIdentity = heloDomain
 			}
-			fmt.Printf("spf result: %q: %q\n", spfDomain, spfStatus)
+			fmt.Printf("spf result: %s: %s\n", spfDomain, spfStatus)
 		}
 	}