fix: makes pouch stop command support timeout value 0

[staugust]

Ⅰ. Describe what this PR did

Make pouch stop -t 0 ${cid} behaves as its help message describes, that is to say stop container immediately.
See https://github.com/moby/moby/blob/68bec0fcf7a5eeb59c027287d06598098edc9f2c/daemon/stop.go#L71

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Here's command to verify it:

cid=$(pouch create -it --net=host --entrypoint=cat ubuntu:bionic)
pouch start ${cid}
time pouch stop -t 0 ${cid}

For now, pouch stop costs about 10 seconds to stop the container that doesn't response to sig-term. With this commit, pouch stops container immediately.

Ⅴ. Special notes for reviews

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[cxz66666]

You can have a look at ProbeContainer function, if timeout is 0, it will never return errtypes.ErrTimeout message, but you can see ctrd/containerd.go L455, which only call SIGKILL when get errtypes.ErrTimeout.
In other words, if just delete this code directly, it will never delete those containers which ignore SIGTERM singal!

[cxz66666]

I already add a pr which add kill command for pouch, after it merge you can simply use pouch kill [id] to stop a container immediately!
So I will close this PR