Allow other modules to know if alias is pathauto-generated via core path hooks

Currently in _pathauto_set_alias() we use:

$path = array('source' => $source, 'alias' => $alias, 'pid' => $pid, 'language' => $language);
path_save($path);

If we use this:

$path = array('source' => $source, 'alias' => $alias, 'pid' => $pid, 'language' => $language, 'pathauto' => TRUE);
path_save($path);

Modules that implement hook_path_save() can check for !empty($path['pathauto'])). Nothing that affects our own module, but helps others.