Add cpu_openbsd

[mguegan]

This PR will add support for OpenBSD CPU statistics. I have duplicated the cpu_freebsd code and tried to handle the main differences between both OS.

This was tested on 6.2R :

• 1 CPU / KVM
• dedicated hardware 4 CPUs

@SuperQ

[discordianfish]

Is there a way without that much code duplication?

[mguegan]

@discordianfish I think it should be possible by sharing everything in common on both OSes into a common file. Maybe it could be done on a second pass in order to clean up all the mess around like #547 which has a much better approach than this one.

[discordianfish]

Not sure if relying on @derekmarcotte to clean up stuff is a good pattern :)
So since this here isn't a fix where I would be open to "clean up later" but a new feature, we should try to get it right the first time.

[zachfi]

Anything else needed here? I'd love to be able to run node_exporter on my OpenBSD boxes.

[discordianfish]

To be clear: Happy to merge this but first the common bits should be moved to a file that is shared with other freebsd.

[SuperQ]

Sorry, we ended up going with #805, we still need to do some cleanup of code duplication in the *BSD collection.