Environment.OSVersion.Platform returns PlatformID.Unix on macOS instead of PlatformID.MacOSX, which is consistent with Mono's behavior.

This is already confusing for Xamarin.Mac developers and I think it's going to be confusing for .NET Core developers as well.

(I wish we hadn't added PlatformID.MacOSX for Silverlight, and just used PlatformID.Unix like Mono.)

To help mitigate the confusion, should PlatformID.MacOSX be marked as obsolete, pointing developers to use PlatformID.Unix or RuntimeInformation.IsOSPlatform(OSPlatform.OSX) instead?

And/or hidden from intellisense?

cc: @terrajobst, @KrzysztofCwalina, @weshaggard, @stephentoub

For reference:

    public enum PlatformID
    {
        Win32S = 0,
        Win32Windows = 1,
        Win32NT = 2,
        WinCE = 3,
        Unix = 4,
        Xbox = 5,
        MacOSX = 6
    }