[ic] 5.3.1 - Out of stock products

[Andreas Grau]

Out of stock items can be preordered, ie. put into the cart using a 
quantity of zero (enforced). Zero quantity items are, however, 
completely removed from the cart later, leaving no trace of preordered 
items.

I am tempted to suggest that there should be no automatic removal for 
zero quantities, but only for qty < 0. This would leave it open to the 
catalog to mark and handle preorders.

lib/Vend/Cart.pm may introduce a pack()-sub to kick zero quantities out 
of the cart if needed. And a catalog.cfg compatibility flag may change 
toss_cart() to old behaviour as well.