Draft: Work on adding support for other UNIXes

[muhammadn]

This is a work in progress until it is agreed that some or most of the support is complete.

So far as discussed here, there is some work on tests to be done.

Since we agree upon support for the three major BSDs (Open/Net/Free), I think those would be prioritised, however, i would like to add a Linux port as well since this would also have an added benefit.

Darwin/macOS support will be deprioritized so i would have more time to work on the above first, since adding Darwin support would be a challenge without the complete macOS/Darwin source code from Apple and requires porting code from other BSDs.

Comments are welcomed.

[smytht]

Thanks for this Pull request, I like effort and would love to see it come to fruition , there are some code structure / project management considerations here and I'm not experienced enough as a programmer to OK this, I do agree with prioritising one or 2 other platforms in addition to OpenBSD to see how the code could be made more manageable for multiple OS
I like the idea of trying FreeBSD support first to see what needs to be changed in the code base to make generic *BSD and derivatives support easier.