Werk #6213: systemd_units.services: Introduce Monitoring of Systemd Services

A new check is introduced that checks linux services using 'systemctl --all'. The check reports the 'active' attribute of the discovered services and their description. The state is configurable.

Also a summary of the linux services is given. That check reports the number of all services and for each service type, i.e. 'active', 'inactive', 'failed' and other ones.

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