update DiskSpace plugin to track status per filesystem

This plugin monitors multiple filesystems, but the status of each filesystem was not being tracked separately.  So if two filesystems were mounted and only one of them had a problem, the Status would flop between a problem state and OK.

So the plugin now makes use of the new wubot_status_key (which was just added for this purpose) to keep track of the status per filesystem.

The monitor also now logs a message to stdout when there is a disk space problem.