Updates the /cabnavigator.php in two respects, zero rack unit drop, flags appear in legend only if used

Updates the /cabnavigator.php in two respects, zero rack unit drop, flags appear in legend only if used. 

The first screenshot shows the original behavior. [1] shows all potential flags used for devices in the cabinet view. If a zero height device is present then rack position 0 is shown for a rack as can be seen at [2]. Since zero height devices already appear in the extra box "[Zero-U Devices]" it doesn't make much sense to generate the zero rack unit.
![cabinet_height_fixed_prior2patch_20130617a](https://f.cloud.github.com/assets/2499906/664288/73d0d184-d781-11e2-94b0-bbc9597e5024.png)

The fix can be seen in the following picture. Only the flags which are actually used in the rack view are displayed in the legend, see [1]. Also the zero rack position has been removed from the rack view, see [2].
![cabinet_height_fixed_20130617a](https://f.cloud.github.com/assets/2499906/664295/a50ae69a-d781-11e2-9ff3-3cb13d89cae3.png)