[HTML5] Close IDBFS database on exit.

This should be made available in emscripten in a decent way.
Possibly after unmount, to free the database lock and allow performing
operations on it from javascript after the Emscripten Runtime has
exited.

platform/javascript/engine/engine.js

@@ -121,6 +121,7 @@ Function('return this')()['Engine'] = (function() {
 			me.rtenv['noExitRuntime'] = true;
 			me.rtenv['onExecute'] = me.onExecute;
 			me.rtenv['onExit'] = function(code) {
+				me.rtenv['deinitFS']();
 				if (me.onExit)
 					me.onExit(code);
 				me.rtenv = null;