Independent invoice number

for osCommerce Online Merchant v2.2

Independent invoice number v1.0, for version 2.2-ms2-060817. This contribution generates invoice numbers independent of order number, making possible to delete/reject some orders and keep at the same time correlative invoice numbers (a must to most account departments). It also uses a customizable invoice serial prefix, to distinguish OScommerce invoices from those from other origins. This contribution adds 2 fields to the `orders` table, and as such can be considered problematic. However, it doesn't seem to conflic with the existing vanilla codebase. If unsure, be cautious: Backup your files and use a testing installation.