Apply gateway group name change through config

Fixes #2914 by applying 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. I put the functionality in a routine in gwlb.inc because then it could be easily called from anywhere in future. It looked like too much special code if patched into system_gateway_groups_edit.php
Note: system_gateway_groups_edit.php is also modified by pull request 499. Either version is standalone good, the 2 obviously need to merge if both changes are accepted.