Fix issue #251 and others 'undefined array key' errors.

[nerun]

This is a diff between my changes and the file from current master branch. These lines were added. These additions are harmless to the plugin and has no reasons to not add them:

39a40,46
>         
>         if ( !isset($USERINFO['grps']) ){
>             $USERINFO['grps'] = '';
>         }
>         if ( !isset($INFO['id']) ){
>             $INFO['id'] = '';
>         }
72a80,82
>         if ( !isset($JSINFO['move_renameokay']) ){
>             $JSINFO['move_renameokay'] = '';
>         }

Issue #251

[michitux]

It doesn't seem like a good idea to just write to these global variables, in particular as some of them shouldn't be of type string. Instead, the code should use proper ways to test if the array key exists before using it. I've just pushed a commit that fixes the remaining problems (one was already fixed).