Fix CPUVulnerabilities() reporting from sysfs

[mwasilew2]

closes #531

This PR contains a few changes (each in a separate commit):

• adds fixtures and tests for parsing cpu vulnerabilities from sysfs, fixes the typo in the vulnerability string
• switches to using a map of vulnerabilities instead of a slice (makes working with the result easier, e.g. when testing)
• switches to using encoding of the state
• adds license in the test file
• adds linux build flags
• adds test cases for vulnerable state, brings state string to lower cases (to try and make it case insensitive)

[mwasilew2]

@discordianfish @SuperQ is there anything else I can help with here?

[mwasilew2]

rebased onto upstream

edit: due to unrelated failures in the CI

[mwasilew2]

the codespell job is failing, but it's unrelated to this change

Here's the job definition: https://github.com/prometheus/procfs/blob/abbfda0b3c04c4536faa158b44a4047e357ede23/.circleci/config.yml#L38-L45

the docker image hasn't changed recently: https://hub.docker.com/r/circleci/python/tags

codespell pip package was updated yesterday: https://pypi.org/project/codespell/#history

I think this is unrelated and should be handled in a separate issue/PR. I'll open a separate one

[SuperQ]

Going to ignore the codespell, it's fixed in HEAD.