Call FS.syncfs on Emscripten (#838)

Also, add a sync parameter to avoid calling syncfs too often.

Calling syncfs twice in a row is both inefficient and leads to errors
displaying twice. This allows us to bypass it when saving unlock.vvv as
part of savestatsandsettings.

desktop_version/src/Game.h

@@ -156,8 +156,8 @@ public:
 
     void loadstats(ScreenSettings* screen_settings);
 
-    bool savestats(const ScreenSettings* screen_settings);
-    bool savestats(void);
+    bool savestats(const ScreenSettings* screen_settings, bool sync = true);
+    bool savestats(bool sync = true);
 
     void deletestats(void);