Add background mode support to localagent and implement instance check

lib/instance/mutex.go

@@ -28,6 +28,19 @@ func Acquire() (*Guard, error) {
 	return &Guard{handle: h}, nil
 }
 
+func AlreadyRunning() bool {
+	name, err := windows.UTF16PtrFromString(config.MutexName)
+	if err != nil {
+		return false
+	}
+	h, err := windows.OpenMutex(windows.SYNCHRONIZE, false, name)
+	if err != nil {
+		return false
+	}
+	windows.CloseHandle(h)
+	return true
+}
+
 func (g *Guard) Close() {
 	if g == nil || g.handle == 0 {
 		return