cgroup: use proc fd when reading details about current process for explicitness

[Snaipe]

Knowing in which context you're running is hard, when most of the namespaces can be interchanged at any time during init.

Instead of tossing a coin, use file descriptors to /proc that we open early.

[yabberyabber]

This change appears to unblock my container development work. I've just restarted the review in case it was an intermittent failure

[Snaipe]

The failure is just the GH action bit-rotting (sigh). I'll ignore it for now.