Add process package

[nirs]

Current code contains multiple implementations for managing a process
using pids, with various issues:

• Some are unsafe, terminating a process by pid without validating that
  the pid belongs to the right process. Some use unclear
• Using unclear terms like checkPid() (what does it mean?)
• Some are missing tests

Let's clean up the mess by introducing a process package. The package
provides:

• process.WritePidfile(): write a pid to file
• process.ReadPidfile(): read pid from file
• process.Exists(): tells if process matching pid and name exists
• process.Terminate() terminates a process matching pid and name
• process.Kil() kill a process matching pid and name

The library is tested on linux, darwin, and windows. On windows we don't
have a standard way to terminate a process gracefully, so
process.Terminate() is the same as process.Kill().

I want to use this package in vfkit and the new vment package, and later
we can use it for qemu, hyperkit, and other code using managing
processes with pids.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @nirs. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[nirs]

@medyagh this should be used in #20506 and #20501.

[minikube-bot]

Can one of the admins verify this patch?