dotnet 跨平台兼容的 socket::shutdown 函数

1、跨平台兼容

2、避免托管异常降低效率或者炸线程乃至虚拟机

平台兼容：Linux、Windows

函数实现： 

#if NETCOREAPP
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
#endif
        private static void Shutdown(Socket socket)
        {
            if (socket == null || socket.SocketType != SocketType.Stream)
            {
                return;
            }
            PlatformID platform = Environments.Platform;
            if (platform == PlatformID.Win32NT)
            {
                NativeMethods.win32_shutdown(socket.Handle, SocketShutdown.Send);
            }
            else if (platform == PlatformID.Unix)
            {
                NativeMethods.linux_shutdown(socket.Handle, SocketShutdown.Send);
            }
            else
            {
                try
                {
                    socket.Shutdown(SocketShutdown.Send);
                }
                catch (Exception) { }
            }
        }

P/Invoke 声明：

        private static class NativeMethods
        {
            [Dllimport("ws2_32.dll", SetLastError = false, EntryPoint = "shutdown")]
            public static extern SocketError win32_shutdown([In] IntPtr socketHandle, [In] SocketShutdown how);

            [Dllimport("libc", SetLastError = false, EntryPoint = "shutdown")]
            public static extern SocketError linux_shutdown([In] IntPtr socketHandle, [In] SocketShutdown how);

            [Dllimport("libc", SetLastError = false, EntryPoint = "setsockopt")]
            public static extern int linux_setsockopt(int socket, int level, int option_name, IntPtr option_value, int option_len);

            [Dllimport("ws2_32.dll", SetLastError = false, EntryPoint = "setsockopt")]
            public static extern int win32_setsockopt(int s, int level, int option_name, IntPtr optval, int optlen);
        }