Cleanup the form action URLs generated on the product listing pages

Cleanup the form action and other URLs generated on the product listing pages. More specifically stop the generation of duplicate "products_id" parameters.

When a customer adds a product to the shopping cart from a product listing page (with the store set to show the shopping cart) the "continue shopping" (back) button includes the "products_id" parameter. When the customer clicks the "continue shopping" button the customer is sent to the product listing page with "products_id" in the URL. This currently causes generated forms on the product listing page to include the "products_id" parameter twice in the form's action URL (once with the last added product id and a second time with the id of the product to be added).