service: use systemctl list-units instead of list-unit-files

The service module checks for the presence of a service before issuing a command on it, e.g. before starting the service. On systemd systems, the list of available services is currently retrieved with `systemctl list-unit-files`. Because LSB-defined services in /etc/init.d don't have unit files, they are not part of this list. This commit adds an additional call to `systemctl list-units`, to add support for LSB services.