While debugging SDDM, I realised there were some occasions when we were not cleaning up the logind sessions, this showed there were some processes we were not ever telling them to close.
This adds 2 changes fundamentally:
- Makes sure startplasma itself terminates its own processes.
- Allows plasma_session to terminate its own processes.
A good way to test this is to restart a plasma session several times and use loginctl to see how they're listed.