logger: ensure the 'From' header of superadmin notifications uses a FQDN
Before, depending on the configuration of the machine hosting Schleuder, it might have been only a hostname, like "lists". Now, it will be "lists.example.org".
Before, depending on the configuration of the machine hosting Schleuder, it might have been only a hostname, like "lists". Now, it will be "lists.example.org".