API Proposal: Add PlatformID.Unknown value

[marek-safar]

Background and Motivation

With the addition of more platforms to dotnet/runtime the existing .NET Framework APIs are missing way to express them. Instead of duplicating logic between System.Runtime.InteropServices.RuntimeInformation API and old .NET Framework APIs we introduce generic undefined value for old APIs.

Proposed API

namespace System
{
    public enum PlatformID
    {
        MacOSX = 6,
+       Unknown = 7,
    }

[Dotnet-GitSync-Bot]

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[bartonjs]

Video

API review suggests that "Other" is a better name, since it is "known" to not be one of the other values.

namespace System
{
    public enum PlatformID
    {
        MacOSX = 6,
+       Other = 7,
    }

Additionally, if we haven't already done obsoletion plans for this enum and the things related to it, it's probably a good idea.