Allows users to report PMs to admins.

• Accessed at ?action=pm;sa=report;pmsg=x
• Dies with a "no_access" error if the feature isn't enabled
• Uses the 'report_message' sub-template if $_REQUEST['report'] isn't set
• Uses the 'report_message_complete' sub-template if the report has been sent
• Sends the report only to a specific admin if $_REQUEST['ID_ADMIN'] ($_REQUEST['id_admin'] in 2.0) is set