Consider using systemd for pause PID

[saschagrunert]

We added support for handling namespaces in #908, where @cgwalters mentioned that we could consider using systemd for the PID handling: #908 (comment)

We can experiment with the implementation to see how much the resource overhead would be when using systemd.

[haircommander]

I am unsure we need systemd here. ultimately, conmon-rs will still be watching the lifecycle of the pause process. we don't want it to automatically restart, and conmon-rs should outlive the process. I don't see any benefits a systemd unit would add

[cgwalters]

Obviously up to you! The reason I commented here is because pid files are IMO a code smell; anywhere one has a pid file a systemd unit would be more reliable. Specifically reading pid files and invoking kill() on them is inherently racy with the process crashing and pid reuse etc.