Bugfix with recent changes to new/used unit flag setting on purchase unit.

In scenario where unit doesn't exist against the customer and you add it for the first time, the purchase unit screen will scrub unit details at various points making it  impossible to add a new unit for purchase in this screen.  Floor stock purchases unaffected.

Underneath this is a subtle design issue which is now also fixed. Previously c9 may have flipped the new/used flag at random times during process of inputting a unit to purchase. This should no longer be happening either from now on.