Use double underscores in mail id

### Problem/Motivation

http://api.drupal.org/api/drupal/includes--mail.inc/function/drupal_mail/8

 $message = array(
    'id' => $module . '_' . $key, 

should be

 $message = array(
    'id' => $module . '__' . $key, 

That would make it easier to identify and alter emails, and could also avoid possible id collisions.

### Proposed resolution

### Remaining tasks