fix: resolve issues with cgroup2 and lxc4

[glenux]

Here's the patch for lxc >= 4.0.2 that should be able to support both cgroups-v1 and cgroups-v2. It was tested on vagrant 2.2.14 + lxc 4.0.6... and seems to work.

Notes:

• LXC 3.1.0 mounts two entries in cgroups now instead of one.
• according to kernel documentation, the "tasks" file was removed from CGROUPMNT in cgroup-v2, thus we have to use the lxc-info fallback to get the pid.

[fgrehm]

Hey, sorry for the silence here but this project is looking for maintainers 😅

As per #499, I've added the ignored label and will close this issue. Thanks for the interest in the project and LMK if you want to step up and take ownership of this project on that other issue 👋