Werk #13124: Improve usability of single systemd services discovery ruleset

The ruleset "Systemd single services discovery" allows for configuring the discovery of single systemd services. This ruleset has been slightly reworked to improve its usability. However, users do not need to take any immediate action, since current configurations will continue working.

In particular, it is not possible any more to configure empty conditions on the systemd service descriptions, unit names or states. When editing a previously configured rule with empty conditions, users must untick these empty conditions in order to be able to save the changes. Also, any regular expressions are now checked for validity before saving.

Furthermore, we have added a default restriction on the unit names. This restriction excludes the volatile systemd services check-mk-agent@.service used by the Checkmk agent from being discovered. These services appear and disappaer frequently, constantly resulting in vanished and undiscovered services. Therefore, they are never discovered, indepedently of the user configuration. See also werk #12907.

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