Reorganization of boost filesystem usage

- Removed (most of) boost filesystem namespace usings
- Replaced boost::filesystem::fstream with std::fstream and different
constructor that should be available on any plaftorm

client/mainmenu/CMainMenu.cpp

@@ -61,8 +61,6 @@
 #include <SDL.h>
 #endif
 
-namespace fs = boost::filesystem;
-
 std::shared_ptr<CMainMenu> CMM;
 ISelectionScreenInfo * SEL;