Apply gateway group name change through config

This applies a gateway group name change throughout the config, keeping it consistent. It works for me - rules, OpenVPN, IPsec and DynDNS uses automagically change their name.
Note: system_gateway_groups_edit.php is also modified by pull request 499, so I had kept this code waiting for 499 to be dealt with, but Easter happened and so there has been no more comment or action on 499.