Xfce processes are terminated for on-demand dekstop sessions

[djperrefort]

When spawning desktop sessions via on-demand, Xfce launches a number of processes that are immediacy adopted by the init process. By default, shinigami terminates these processes and interrupts the user session.

We want to protect Xfce sessions from termination. A possible solution is to never terminate processes trees where any process in the tree has theXDG_SESSION_ID variable defined. In theory, this should protect any processes tied to a desktop session. The (acceptable) tradeoff is that shinigami may leave a few processes behind that would otherwise be cleaned up. Fortunately, we expect these processes to be low utilization.

Credit to @buttermutter and @iamtroy412 for the proposed solution.