wgengine/router: dns: unify on *BSD, multimode on Linux, Magic DNS (#536)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/router_darwin.go

@@ -52,3 +52,13 @@ func (r *darwinRouter) Up() error {
 	}
 	return r.Router.Up()
 }
+
+func upDNS(config DNSConfig, interfaceName string) error {
+	// Handled by IPNExtension
+	return nil
+}
+
+func downDNS(interfaceName string) error {
+	// Handled by IPNExtension
+	return nil
+}