Per-platform .NET Runtime prompt/dependencies install

[MikhailArkhipov]

Windows/OSX Sierra+ - nothing to do
Earlier than Sierra: needs OpenSSL

Linux: depends on the platform, see
https://docs.microsoft.com/en-us/dotnet/core/linux-prerequisites?tabs=netcore2x
https://github.com/dotnet/core/blob/master/Documentation/prereqs.md
https://github.com/dotnet/core/blob/master/release-notes/2.0/2.0-supported-os.md

POR:
All platforms: If .NET 2+ runtime is present, do nothing
Windows/Sierra+: download prebuilt binaries
Linux: offer runtime install, ex https://www.microsoft.com/net/download/dotnet-core/runtime-2.0.6

Possibly automate (per platform)

[MikhailArkhipov]

No longer needed, download is automatic. #1231