Ideally we should use https://github.com/containers/psgo rather than exec-ing ps, but this refactoring can be done later.

running the ps command in a rootless container with cgroupv1 is not possible because rootless container does not rely on cgroupv1.
the problem does not arise in the case of rootlfull (v1 or v2) && rootless ( v2)
The idea to get around this problem is to join the mount namespace of rootless container when we are dealing with rootless container on cgroupv1 and fetch process info inside the pid mount ns.

FYI : for pid host is like doing a ps on the host and dealing with the host /proc so we should add this to the condition when implementing --pid=host

I prefer making this refactoring later when using psgo rather than exec-ing ps. It handles better this use cases.

Cc @AkihiroSuda

Pid host is implemented here #273
The case of host pid should be handled when refacto with psgo