wgengine/router: remove wireguard-go config from settings.

Instead, pass in only exactly the relevant configuration pieces
that the OS network stack cares about.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/router/router_darwin.go

@@ -26,7 +26,7 @@ func (r *darwinRouter) Up() error {
 	return nil
 }
 
-func (r *darwinRouter) SetRoutes(rs RouteSettings) error {
+func (r *darwinRouter) Set(rs Settings) error {
 	if SetRoutesFunc != nil {
 		return SetRoutesFunc(rs)
 	}