Add a context object in nebula.Main to clean up on error (#550)

tun_windows.go

@@ -24,6 +24,13 @@ type Tun struct {
 	*water.Interface
 }
 
+func (c *Tun) Close() error {
+	if c.Interface != nil {
+		return c.Interface.Close()
+	}
+	return nil
+}
+
 func newTunFromFd(l *logrus.Logger, deviceFd int, cidr *net.IPNet, defaultMTU int, routes []route, unsafeRoutes []route, txQueueLen int) (ifce *Tun, err error) {
 	return nil, fmt.Errorf("newTunFromFd not supported in Windows")
 }