HttpServer: Support for Unix Sockets

[philippseith]

I am trying to use NetCoreServer to create an app for ctrlX OS. ctrlX OS secures access to app UI with a reverse proxy, but to prevent that local processes can hijack sensitive data, the app should listen on a unix socket instead of a local tcp port.
Currently, Kestrel is the only dotnet webserver able to listen on a unix socket, but NetCoreServer has a smaller footprint, can be published trimmed and even AOTed.
By adding one c'tor for HttpServer and TcpServer and a one-liner in TcpServer.CreateSocket, this capability can be added to NetCoreServer.

I was able to test the functionality successfully on Linux with the HttpServerUnixSocket sample, nginx and a browser. On Windows, I did just a simple curl test (and was surprised that Windows is able to create something like a unix socket :-)), which worked, too.