logind: rework session shutdown logic

Simplify the shutdown logic a bit:

- Keep the session FIFO around in the PAM module, even after the session
  shutdown hook has been finished. This allows logind to track precisely
  when the PAM handler goes away.

- In the ReleaseSession() call start a timer, that will stop terminate
  the session when elapsed.

- Never fiddle with the KillMode of scopes to configure whether user
  processes should be killed or not. Instead, simply leave the scope
  units around when we terminate a session whose processes should not be
  killed.

- When killing is enabled, stop the session scope on FIFO EOF or after
  the ReleaseSession() timeout. When killing is disabled, simply tell
  PID 1 to abandon the scope.

Because the scopes stay around and hence all processes are always member
of a scope, the system shutdown logic should be more robust, as the
scopes can be shutdown as part of the usual shutdown logic.

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