Returns $arg or the user ID of the current user if $arg is '%' or empty.
@todo rethink the naming of this in Drupal 8.
function user_uid_optional_to_arg($arg) {
// Give back the current user uid when called from eg. tracker, aka.
// with an empty arg. Also use the current user uid when called from
// the menu with a % for the current account link.
return empty($arg) || $arg == '%' ? $GLOBALS['user']->uid : $arg;
}