Fix cookie generation and set the correct cookie path for the greeter

This reverts commit fed4519b3fa90f92cef1a4f9802eabc9692508c1 which removed generating a cookie in /var/run/xauth. This caused the X server to point to a non existing cookie (and everywhere else where m_authPath is referenced in the code).

This also fixes XAUTHORITY path when running the greeter under the sddm user, it currently points to /var/run/xauth and this is wrong, the sddm user can't access that directory.

I also fixed a double slash issue in the authDir, there is a slash added in both Configuration.cpp and Display.cpp.

A better solution might be to stop using /var/run/xauth and to always create a single cookie in the /var/lib/sddm directory though, used by the greeter and X, but this would need to make sure the directory exists and might require more changes.

EDIT: Or should m_authPath be removed instead and it'll fall back on $HOME/.Xauthority automatically?