Enable Pod to run pmctl add forward without systemd

[tapih]

What

pmctl add forward now runs with systemd.
https://github.com/cybozu-go/placemat/blob/7c4a2adf2edfb26ca10a3cb817d6e057830d3399/v2/cmd/pmctl2/cmd/forward_add.go#L62

This command cannot be executed inside Pod because the process with PID 0 is not systemd.
In this task, update pmctl to work without systemd.

How

T.B.D.

This functionality is used only when we want to check Grafana dashboard which runs on placemat.
If you think it's not necessary and executing ip netns exec ... is enough, you can remove this subcommand and please be sure to make documentation for it.

Checklist

• Finish implementation of the issue
• Test all functions
• Have enough logs to trace activities
• Notify developers of necessary actions