Systemd provides the status of a given user and system session as a
human readable string.
Retrieve this information via RequestSystemState and guard the
retrieve of failed units depending on this request.
The functionality is extended but does not change, which means that
failed units in any granularity are displayed as before.
Update documentation in the meantime.
Signed-off-by: Steffen Kothe <steffen.kothe@skothe.net>
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