Add: processmail.pl - Pushes incoming acknowledgement mails to Nagios.

Derived from

   http://www.techopsguys.com/2010/01/05/acknowledge-nagios-alerts-via-email-replies/

Uses procmail recipe to accept mails from whitelisted senders
only.There are two possible notifications -- for host or service
alerts -- for which several actions can be performed: 'ack',
'clear', 'enable', 'disable'. To send an external command on an
alert, reply using any of the following subject lines:

   action Re: ** Host $HOSTSTATE$ alert for $HOSTNAME$! **
   action Re: ** $NOTIFICATIONTYPE$ alert - $HOSTNAME$/$SERVICEDESC$ is $SERVICESTATE$ **