chore: adjust build-tags for non-linux / non-unix platforms

[thaJeztah]

Opening this project in my IDE on macOS shows various types and functions (some of which are imported from moby/sys) as "undefined".

While this project is intended for Linux, it would be good to adjust build-tags to exclude files on other platforms (we do so "partially" for some files that have a _linux suffix.

[BenTheElder]

I just ran into this with Kubernetes's end to end tests.

The tests are importing this library for ConvertCPUSharesToCgroupV2Value to use against remote linux pods/containers, that logic should be portable to all platforms but importing the package causes compilation failures ... for all of the unused code.

Would it be acceptable to split these utils to a cross-OS package, or build tag the other code?

[BenTheElder]

Open question: Currently the project uses _linux.go pattern for some files but not all, renaming a lot of files seems like churn, so build tags then?

Perhaps for Kubernetes's case we should just duplicate this computation, I assume it's meant to be relatively stable ...?

[thaJeztah]

Open question: Currently the project uses _linux.go pattern for some files but not all, renaming a lot of files seems like churn, so build tags then?

I'm personally fine with either; renaming files is usually rather uneventful with git (as it just shows up as a rename), and in some cases it's nice to be able to see that a file is linux-only by just looking at the filename, but build-tags can be more flexible.

The only thing we should try to avoid is to implement stubs for things that will never be supported; I know for some modules we do, but they can become a sliding slope (and sometimes lead to the false assumption that a module can be used on other platforms).

Are there any other utilities that would be made platform-agnostic, or it's purely this one?

@kolyshkin any preference?