Satisfy linters

config/config.go

 	publicIPv6 net.IP, publicIPv6Port uint16,
 	statsIP net.IP, statsPort uint16,
 	secret, adtag string) (*Config, error) {
-	if strings.HasPrefix(secret, "dd") {
-		secret = secret[2:]
-	}
+	secret = strings.TrimPrefix(secret, "dd")
 	if len(secret) != 32 {
 		return nil, errors.New("Telegram demands secret of length 32")
 	}