Only use ids.whitelist.custom if the file is readable.

Fixes an issue I had where the ids.whitelist.custom file existed but was unreadable by the ispconfig user (yes, local mistake, but easy to check) - this situation effectively disabled both the custom and the stock ids whitelist, and when I saved a Server Config > Jailkit section, the IDS tripped.

Note I ended up with an empty or invalid server.config table entry about this time, I guess (but am not 100% sure) that this may have been the cause. Thank you again for the ability to roll back data log changes. :)