Preserve attributes from product form when redirecting anonymous users to node checkout form after logging in

Issue Summary

When anonymous users configure a product and then click 'Add to cart', they are redirected to the login form and then subsequently to the node checkout form. This works well however their attribute selections are not preserved during the redirect process.

The code to do this is already present in uc_node_checkout_add_to_cart_submit() for logged un users, so the attached patch is mostly just copy/pasting snippets from that function to have this done while redirecting anonymous users.