gdm-{wayland,x}-session: protect more session variables
since commit 2802e92d we allow users to override the session environment with systemd's environment.d mechanism.
That is great for PATH, but not so great for XDG_CURRENT_DESKTOP and other potentially lingering session variables.
This commit adds more variables to the blacklist of things that aren't allowed to be preempted by systemd's environment.
Closes #505 (closed)