This patch enables a hook for manipulating exclude addresses for a 
vacation rule.

Rationale: Exclude vacation messages to adresses like root@domain, 
apache@domain etc.

A hook function might be:
     /**
      * Set the exclude addresses used for the vacation module.
      *
      * @param string $user  The username.
      * @param array $value  The default/current value.
      *
      * @return array  A list of exclude addresses.
      */
     public function vacation_excludes($user = null, $value = null)
     {
         $default = array('root@domain', 'apache@domain');
         if (is_array($value)) {
             return array_unique(array_merge($value, $default));
         }
         return $default;
     }