minikube should fail early if executed as root

[tstromberg]

This is a bad practice, and could address some awkward connectivity and authentication issues, like #2548

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[SimonBaeumer]

@tstromberg Hey Thomas!
First of all thank you and the team for this great project!
I noticed that you added the help wanted image in the README.md and would like to start working on this issue, and hopefully some more 👍

The implementation in my head would be to check if minikube was started as root and exit it with an error message.
And some questions:

• Would it make sense to allow root execution if a flag is passed to minikube, i.e. --root?
• Is preventing administrator execution on windows a thing?

[tstromberg]

@SimonBaeumer - I agree. It would be preferable to have an escape hatch, like --running-as-root-is-dangerous.

I'm not really worried about Windows in this case, it doesn't have the side effect of being very confusing about profile ownership.

We'll need to get #3760 resolved first.