Checking for service install on systemd

[annamel]

One of the common ways to check whether the service is installed is getting its status and checking for error code

	_, err = s.Status()
	return err != service.ErrNotInstalled

Unfortunately, for systemd init system such an approach doesn't work.
Status function of Kardianos Service project raises "systemctl is-active" call https://github.com/kardianos/service/blob/master/service_systemd_linux.go#L198
systemctl is-active would return "inactive" status for both stopped and non-existent service. So if the service has never been installed on the machine, Kardianos service.Status() would return Stopped.

It looks like

• either Status() of Kardianos.Service in the case of SystemD should rely on "systemctl status" or "systemctl list-units"
• or there should be some other way in Kardianos to check whether the service is installed