core.lua: Improve setup of cgroups hierarchies, should fix issue #45

- Reuse or create symbolic links if hierarchy is mounted under
  unexpected mount point.

- Handle multiple subsystems in a single hierarchy.

  = If multiple subsystems, which ulatencyd uses, share the same
    hierarchy disable all but the first subsystem.

- Revert changes if mounting of private hierarchy fails.
- Handle more errors and non-standard situations.
- More verbose yet uncluttered log messages.