Add a 'ProductsSpecials' handler

[lat9]

... as requested on the Zen Cart support thread.

[lat9]

Design-detail notes:

1. The Specials export will contain all fields of the store's specials table (minus the specials_id) as well as the associated product's name, 'base price' and model-number. The product's name will appear in the language current at the time the report is generated.
   • Those extra fields are provided only for reference and are not used for a Specials import.
2. Only one specials record is allowed per products_id.
3. On an import, a product's special price can be identified either as a decimal-value, e.g. 59.99, or as a percentage, e.g. 50%.
   • If the value is specified as a percentage, the value leading to the final % must be in the range 0-100.
   • A percentage-off special is applied to the product's base-price and stored as the full specials' price.
   • Unless MODULE_ORDER_TOTAL_GV_SPECIAL is defined, i.e. the ot_gv order-total is installed, and that constant is set to 'true', products that are gift-vouchers cannot be placed on special.
4. The Specials import can include an optional v_dbio_command column. A product's special pricing information will be removed from the database if this column is set to REMOVE for a product.