Create a mapping of OS version to XNU version to variants available

[andrewdavidmackenzie]

Ass Apple updates darwin, it adds some new variants (flavors) available to some API calls (e.g. proc_pidinfo()).

Create a mapping of XNU version --> What variants available --> What versions of mac os support them

So we can make conditional code on OS versions, and maybe also allow to detect the OS version at runtime and hence check the flavors that can be used in some api calls.