Fix losing number of line when wrap the log function

main.go

@@ -23,9 +23,11 @@ var config struct {
 	UDPTimeout time.Duration
 }
 
+var logger = log.New(os.Stderr, "", log.Lshortfile|log.LstdFlags)
+
 func logf(f string, v ...interface{}) {
 	if config.Verbose {
-		log.Printf(f, v...)
+		logger.Output(2, fmt.Sprintf(f, v...))
 	}
 }