Add windows emulation for RTLD_DEFAULT

I'm not sure if calling GetModuleHandle in static construction is safe.
But I assumme it is and works correctly.

There is still potential issue that documentation can be understood
meaning that the HMODULE will resolve only symbols from exe while
RTLD_DEFAULT resolves all global symbols.

library/PlugLoad-windows.cpp

@@ -41,8 +41,7 @@ distribution.
  */
 namespace DFHack
 {
-    //! \todo What windows HMODULE matches RTLD_DEFAULT?
+    DFLibrary* GLOBAL_NAMES = (DFLibrary*)GetModuleHandle(nullptr);
-    DFLibrary* GLOBAL_NAMES = 0;
     DFLibrary * OpenPlugin (const char * filename)
     {
         return (DFLibrary *) LoadLibrary(filename);