wgengine/router: take a link monitor

Prep for #1591 which will need to make Linux's router react to changes
that the link monitor observes.

The router package already depended on the monitor package
transitively. Now it's explicit.

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

cmd/tailscaled/tailscaled_windows.go

@@ -168,7 +168,7 @@ func startIPNServer(ctx context.Context, logid string) error {
 		if err != nil {
 			return nil, fmt.Errorf("TUN: %w", err)
 		}
-		r, err := router.New(logf, dev)
+		r, err := router.New(logf, dev, nil)
 		if err != nil {
 			dev.Close()
 			return nil, fmt.Errorf("router: %w", err)