Adding a gateway from interfaces.php does not work
After pressing the Add button, actually the new gateway is not created.
It uses AJAX to system_gateways_edit but the code there now expects $_POST['save'] to be set.
Fix #7452 save new IPv6 gateway
This is also needed, just like for IPv4 in commit https://github.com/pfsense/pfsense/commit/f3278171b199062279225631903685e608285d3a