Improve CGroup path-to-unit/slice/etc decoding

[netbsduser]

Decoding of CGroup paths into units, slices etc should be improved - right now, for example, if one runs InitWare under systemd using delegation, then the CGroup decoding will give you the systemd unit under which InitWare is running, rather than the InitWare unit.

One approach we could take is to look at unit names in reverse order of components rather than in forward order, as they currently do. This would require some care to avoid confusions.

Another option could be to require that a system-mode scheduler should always run when running InitWare under delegation from systemd, and any user managers run under that one. The system-mode scheduler could publish a file in /var/run/InitWare with its root CGroup path which would be removed before we attempt to resolve CGroup paths to units. This way has the merit of letting us keep these decoding functions with little alteration.