_quote() function clashes with node_assist

Seeing as node_assist is still a requirement, the _quote() function in addnode clashes with the one in node_assist. Here's a replacement that verifies if the function exists first.

// avoid conflicts with other modules
if (!function_exists('_quote')) {
  function _quote(&$item, $key) {
    $item = "'$item'";
  }
}

For completeness sake it also needs to be changed in node_assist.

FYI the patch was created using svn from our local import of the code, but it will probably be quicker to just use the code above.