Change FAPI _submit() callback's optional return array to be associative

Related to http://drupal.org/node/84250, it would be better to have the optional return array like array('path' => 'some/path/here', 'fragment' => 'spot'); instead of array('some/path/here', NULL, 'spot'); or whatever. If that patch goes in, we should be consistent and others agreed that an associative array is more inline with the rest of the data constructs in Drupal.