No default argument values in fieldgroup_nodeapi()

Even though the implementation of hook_nodeapi() in fieldgroup only performs an action for $op == 'view', which uses the last two arguments ($teaser and $page), the function may be called for any other $op, that may or may not use said last arguments.

This causes a PHP error whenever a third party module calls module_invoke_all('nodeapi', $node, $op); without the last two parameters.

The attached patch adds the proper default arguments, as per the Drupal API.