runk: implement the unimplemented features

[liubin]

@ManaSugi has created runk as a container runtime, it works only with some sub-commands that are not unimplemented.

This issue is used to track these unimplemented sub-commands.

basic features

• [x] runk: support ps sub-command #4361
• [x] runk: support list sub-command #4362
• [x] runk: support exec sub-command #4363
• [ ] runk: support Systemd Cgroups #4369
• [ ] runk: support update sub-command #4365
• [ ] runk: support Rootless #4372
• [ ] docs: add to runtime spec implementations

enhancement features

• [x] runk: support pause/resume sub-command #4364
• [ ] runk: support events sub-command #4366
• [x] runk: support init sub-command #4367
• [ ] runk: support checkpoint/restore sub-command #4368
• [ ] runk: support pivot_root #4370
• [x] runk: support SELinux #4371

[rata]

The "features" sub-command will be nice to support to.

It is becoming increasingly important for user namespace adoption to see if the OCI runtime support idmap mounts (via the features subcommand we check that). For more details, see here: https://github.com/opencontainers/runtime-spec/pull/1219. We are enforcing the OCI runtime supports idmap on containerd and crio for userns support.