Fix -debug command line option misbehaviour: forbid stdin redirection to a pipe when run as plugin server

main.go

@@ -112,7 +112,10 @@ func wrappedMain() int {
 	log.Printf("Built with Go Version: %s", runtime.Version())
 
 	// Prepare stdin for plugin usage by switching it to a pipe
-	setupStdin()
+	// But do not switch to pipe in plugin
+	if os.Getenv(plugin.MagicCookieKey) != plugin.MagicCookieValue {
+		setupStdin()
+	}
 
 	config, err := loadConfig()
 	if err != nil {