gdm-{wayland,x}-session: protect more session variables

Closed Ray Strode requested to merge wip/oh-god-environment-variables-again into master

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)

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