There's some low hanging fruit when it comes to initialization of Cgroups. We can do the following:

1. Get rid of `#define`s and upgrade to a `CG_INFO` enum, with some helpers
2. Make `CgroupInfo` RAII, ridding us of the `cleanup` function
3. `set_subsystem_path` is **only** used at construction time, only call it at that time and make those functions private. Also remove `virtual`ness, as this is unnecessary now.