Auto-suggest doesn't work for global users

The auto-suggest doesn't work while administering local groups in a sub-wiki.

This happens because the value of the select is not taken into account.
The xwiki.js code is hardcoded to use only the 'local' mode("wiki=local"):

"xpage=uorgsuggest&classname=XWiki.XWikiUsers&wiki=local&uorg=user&"