cmd/tailscaled, wgengine{,/netstack}: add netstack hybrid mode, add to Windows

For #707

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/subnet_router_wrapper.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package netstack
+
+import (
+	"reflect"
+
+	"tailscale.com/wgengine"
+	"tailscale.com/wgengine/router"
+)
+
+func init() {
+	wgengine.NetstackRouterType = reflect.TypeOf(&subnetRouter{})
+}
+
+type subnetRouter struct {
+	router.Router
+}
+
+// NewSubnetRouterWrapper returns a Router wrapper that prevents the
+// underlying Router r from seeing any advertised subnet routes, as
+// netstack will handle them instead.
+func NewSubnetRouterWrapper(r router.Router) router.Router {
+	return &subnetRouter{
+		Router: r,
+	}
+}
+
+func (r *subnetRouter) Set(c *router.Config) error {
+	if c != nil {
+		c.SubnetRoutes = nil // netstack will handle
+	}
+	return r.Router.Set(c)
+}