src/utils_threshold.c: Fix creation of percentage notifications.

src/utils_threshold.c: Fix creation of percentage notifications.

Joey Hess has reported a problem when creating notifications from
percentage thresholds. Because the (percentage) minimum value is
compared to the (raw) DS value, the following message is possible:

  Message: Host XXX, plugin df type df (instance root): Data source
  "free" is currently 1773072384.000000. That is above the warning
  threshold of nan%.

A new section will handle this case correctly. In the inverted case, the
problem should not exist.