Werk #13230: systemd_units: Handling of 'deactivating' services

Previously systemd services in the 'deactivating' state immediately led to a critical Checkmk service. Now it is ok for any systemd service to be in this state once, or for a configured interval.

Note that this behaviour in particular affected users who monitored their hosts from two Checkmk sites, as the check_mk_agent would be 'deactivating' from time to time (which is expected).

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