If someone wants to extend or reuse Cgroup detenction in the future they
can move `cgroup_linux.go` to `cgroup.go` and add the relevant build
tags.
Requested by: @dadgar
Change the logic from `!linux` to an empty build tag so that *if*
another platform picks up Cgroups support they can add themselves to the
necessary build tags for this fingerprinter and be on their way.
Because this technology isn't inherently Linux-specific and isn't
mutually exclusive of other resource isolation containers, resist the
urge to rename the Cgroup fingerprinter to something generic like the
ResourceContainerFingerprinter.
