Use wininet API to setup system proxy

Get rid of tricky registry handling

Several files come from https://code.msdn.microsoft.com/windowsapps/CSWebBrowserWithProxy-c8535715/view/SourceCode

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

shadowsocks-csharp/Util/SystemProxy/NativeMethods.cs

@@ -0,0 +1,36 @@
+/****************************** Module Header ******************************\
+ Module Name:  NativeMethods.cs
+ Project:      CSWebBrowserWithProxy
+ Copyright (c) Microsoft Corporation.
+ 
+ This class is a simple .NET wrapper of wininet.dll. It contains 4 extern
+ methods in wininet.dll. They are InternetOpen, InternetCloseHandle, 
+ InternetSetOption and InternetQueryOption.
+ 
+ This source is subject to the Microsoft Public License.
+ See http://www.microsoft.com/opensource/licenses.mspx#Ms-PL.
+ All other rights reserved.
+ 
+ THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, 
+ EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED 
+ WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A PARTICULAR PURPOSE.
+\***************************************************************************/
+
+using System;
+using System.Runtime.InteropServices;
+
+namespace Shadowsocks.Util.SystemProxy
+{
+    internal static class NativeMethods
+    {
+        /// <summary>
+        /// Sets an Internet option.
+        /// </summary>
+        [DllImport("wininet.dll", CharSet = CharSet.Ansi, SetLastError = true)]
+        internal static extern bool InternetSetOption(
+            IntPtr hInternet,
+            INTERNET_OPTION dwOption,
+            IntPtr lpBuffer,
+            int lpdwBufferLength);
+    }
+}