Add 'Do not add duplicate' functionality to uc_cart_links.

UC Cart Links did (and may again - #881752: Cart Links - Missing code for "Empty cart" function) have the functionality to empty the cart before adding a given product. I'd like to extend that functionality to allow a new attribute for cart links (-?) where the functionality would say "Don't necessarily empty the cart, but only allow one of this product in there."

The functionality would thus cause the cart link to add one and only one of a given product.

This thought comes from a use-case where we've had confused users clicking a cart link more than once and having to remove a product. Note that this functionality is related to http://drupal.org/project/uc_restrict_qty , and, though the use cases overlap, they're not identical.