Lock down the search path for Windows DLL loading.

At least on systems providing SetDefaultDllDirectories, this should
stop PuTTY from being willing to load DLLs from its containing
directory - which makes no difference when it's been properly
installed (in which case the application dir contains no DLLs anyway),
but does if it's being run from somewhere uncontrolled like a browser
downloads directory.

Preliminary testing suggests that this shouldn't break any existing
deliberate use of DLLs, including GSSAPI providers.

windows/window.c

@@ -347,6 +347,8 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
     HRESULT hr;
     int guess_width, guess_height;
 
+    dll_hijacking_protection();
+
     hinst = inst;
     hwnd = NULL;
     flags = FLAG_VERBOSE | FLAG_INTERACTIVE;