Add systemd service file
Created by: SunilMohanAdapa
- When running in an environment without systemd, the changes are completely ignored.
- When running under systemd, the patch introduces a systemd native service file to take advantages of nice feature provided by systemd.
- One of the feature currently taken advantage of is the ability to restart the service automatically if it ever crashes or exits with error.
- Another feature is that when we wish to kill Plinth all the processes under the cgroup are killed as well.