Resetting core client configuration not possible in some cases

[smasch]

Setting options with the cc_config.xml works fine, but to unset options the only way is to explicitly set them to their default value. This makes it difficult to manage different clients with add-ons like BoincView? and resetting configurations via deletion of the xml file is impossible without restarting the client.

I already had emailed David weeks ago and also send a patch to the alpha list - ​link - but without response ... please include a fix for 5.10

[davea]

I checked in the changes.