[release-1.22] Properly handle operation as init process

[brandond]

Proposed Changes

Properly handle operation as init process. This includes both vacating the root cgroup on cgroupv2 systems, and reaping child processes since containerd doesn't clean up after its own shims.

This is a Go adaptation of the behavior proposed in #3237; we rejected doing this in shell and K3d has been using a workaround ever since.

Types of Changes

bugfix

Verification

• Run K3s in Docker on a system with cgroup v2; note that it works
• Note that there are no <defunct> processes inside the Docker container

Linked Issues

• Backport of Properly handle operation as init process #4086
• K3s docker image cannot be run on a cgroup v2 host #4085
• Zombie processes in k3s running through docker-compose #2722

User-Facing Change

The K3s docker image now works on cgroup v2 systems, and properly reaps terminated containerd shim processes.

Further Comments

[brandond]

Failed on a CI flake.
E0928 06:16:23.627678 55 kubelet.go:1406] "Failed to start cAdvisor" err="inotify_init: too many open files"