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.

smtp/path.go

@@ -1,6 +1,7 @@
 package smtp
 
 import (
+	"strconv"
 	"strings"
 
 	"github.com/mjl-/mox/dns"
@@ -22,6 +23,24 @@ func (p Path) String() string {
 	return p.XString(false)
 }
 
+// LogString returns both the ASCII-only and optional UTF-8 representation.
+func (p Path) LogString() string {
+	if p.Localpart == "" && p.IPDomain.IsZero() {
+		return ""
+	}
+	s := p.XString(true)
+	lp := p.Localpart.String()
+	qlp := strconv.QuoteToASCII(lp)
+	escaped := qlp != `"`+lp+`"`
+	if p.IPDomain.Domain.Unicode != "" || escaped {
+		if escaped {
+			lp = qlp
+		}
+		s += "/" + lp + "@" + p.IPDomain.XString(false)
+	}
+	return s
+}
+
 // XString is like String, but returns unicode UTF-8 domain names if utf8 is
 // true.
 func (p Path) XString(utf8 bool) string {