Prevent access to the root file system via the memorycard mount points

Before any memorycard is used the mount point exposes "/". Let's
introduce a special value "(empty)" to allow a DIR mount to point to
nothing.

src/MemoryCardManager.cpp

@@ -287,7 +287,7 @@ MemoryCardManager::MemoryCardManager()
 	 * until it's mounted, anyway. */
 	FOREACH_PlayerNumber( pn )
 	{
-		FILEMAN->Mount( "dir", "/", MEM_CARD_MOUNT_POINT_INTERNAL[pn] );
+		FILEMAN->Mount( "dir", "(empty)", MEM_CARD_MOUNT_POINT_INTERNAL[pn] );
 		FILEMAN->Mount( "timeout", MEM_CARD_MOUNT_POINT_INTERNAL[pn], MEM_CARD_MOUNT_POINT[pn] );
 	}
 }