Add an exit value argument to the command channel shutdown command

With #1108 (closed), Kea servers will do a graceful shutdown if a db cannot be recovered. However, this causes an exit status of 0 rather than something non-zero (the "standard" for failed process). Some users may object to this. If we add an exit value argument to the shutdown command, we could readily set it accordingly. In addition, users could use this mechanism to customize their own operations, by assigning their own meaning to their own exit values.