Check the entire scope chain before concluding a 'for' var is global.

This should fix #1016. The question is whether there are other similar bugs. I didn't notice any other likely error messages, so maybe not.

```bash
cat > check-global-var-in-for.js <<"END"
(function(){
    'use strict';
    var myList = [];
    function something(){
        var key;
        function sub(){
            for(key in myList){}
        }
    }
})();
END
./bin/jshint check-global-var-in-for.js
```