Some exit handling changes to KDE units to make shutdown more robust.

- Akonadi: consider 255 a "success" exit code
  Reason: after issuing a "akonadictl stop", main process exits with 255.

- kdeinit: force-terminate process tree after 5 seconds
  Reason: * kdeinit4_shutdown simply does not work in many cases
          * new processes can be spawned in this tree, but they are
            never handled by kdeinit4_shutdown
          * some processes may crash and invoke a crash handler window
            (again, not tracked by kdeinit4_shutdown)
Update user-session@.service unit.
Add gpg-agent.service from gh:zoqaeski/systemd-user-units.