Fix cgroups detection

[agners]

Fixes: #107

[agners]

FWIW, the const definition with 0 makes all three consts 0:

const (
	CGroupUnknown CGroupVersion = 0
	CGroupV1
	CGroupV2
)

So this lead to the situation where always CGroupV2 code path has been used 🥶 .

Using iota identifier tells go to increment each value by 1 (essential create an enum).