unbound: restart on new wan IP

not very good, it bounces the unbound cache, the listening or sending
interface is probably not optimal on an OpenVPN...

src/etc/inc/plugins.inc.d/unbound.inc

@@ -41,6 +41,7 @@ function unbound_configure()
     return array(
         'dns' => array('unbound_configure_do'),
         'hosts' => array('unbound_hosts_generate:0'),
+        'newwanip' => array('unbound_configure_do'),
     );
 }