[patch] Use of more than one coupon of same discount for fixed price off

We have fixed $20/$50/$100 off discounts setup that use a code to trigger them on any product. They are essentially gift vouchers, but this module does a much better job of handling the line items and talking to paypal than any of the others I have found dedicated to the task. I can also setup enticement coupons with this module much more easily.

This patch allows properly configured discount instances to have any number of one type of discount coupon included as a discount. Including to the point of going negative. I'm working on another sub-module that will give "change" in a specially generated discount instance so this was not taken into account. Negative ballances + the uc_free_order works well.

eg. 2x $20 discount coupon codes, will only give you a $20 discount in 6.x-2.4. With this patch, it goes to $40.

Maximum applications: 0
Max uses: 0
Max uses per user: 0
Max uses per code: 1
Maximum applications on fixed amount off OK: ticked