rc.lua: Prevent clients from being lost on RANRD changes (FS1155)

Testing done:

* Using nvidia-settings

Why:

* Necessary to get Awesome 3.5 into Ubuntu and Debian unstable (Jessie + 1). Too late for Jessie.

I know it is less clean than the branch that support RANDR, but this is a short term fix for the 3.5 branch