expose listen Socket property in Interface ISocket.

[wuqiong]

some times, need to get the underlying listen socket for other purpose. as I need to prevent inherit the listen socket when creating subprocess.

[DllImport("kernel32.dll", SetLastError = true)]
static extern bool SetHandleInformation(IntPtr hObject, uint dwMask, uint dwFlags);
private const uint HANDLE_FLAG_INHERIT = 1;
// ...
private static void MakeNotInheritable(Socket tcpListenedSocket)
{
    var handle = tcpListenedSocket.Handle;
    SetHandleInformation(handle, HANDLE_FLAG_INHERIT, 0);
}

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