Add support of global state to systemd.unit.info and systemd.unit.get

As of 2022-06-13 Zabbix Agent2 does not have ability corresponding to command systemctl show.
This command shows overall system status rather than unit status and particularly property SystemState that is most important indicator of system operation state.
For example command systemctl show --property=SystemState --value returns literal "degraded" when at least one systemd unit is in failed state. It will be worth to support item keys like systemd.unit.info[,SystemState] and systemd.unit.get[] on a zabbix side.