CiviMail sets X-header "X-CiviMail-Bounce" on outbound bulk email, but does not use it to identify an inbound bounce

Given my particular email system, inbound bounces are not identified as such by EmailProcessor.php. When outbound bulk email is sent from Mailing.php, an X-header (X-CiviMail-Bounce) is already added to each message. This patch enhances EmailProcessor.php to look for this X-header to identify inbound bounces.