Simplify unprintable handling in UnpackDomainName (#845)

* Avoid the for loop for unprintable octets in UnpackDomainName

* Unify unprintable checking with writeTXTStringByte

These constants are identical.

msg.go

@@ -405,14 +405,11 @@ Loop:
 				case '"', '\\':
 					s = append(s, '\\', b)
 				default:
-					if b < 32 || b >= 127 { // unprintable, use \DDD
+					if b < ' ' || b > '~' { // unprintable, use \DDD
 						var buf [3]byte
 						bufs := strconv.AppendInt(buf[:0], int64(b), 10)
-						s = append(s, '\\')
-						for i := len(bufs); i < 3; i++ {
-							s = append(s, '0')
-						}
-						s = append(s, bufs...)
+						s = append(s, '\\', '0', '0', '0')
+						copy(s[len(s)-len(bufs):], bufs)
 					} else {
 						s = append(s, b)
 					}