DX: Refactor uc_node_checkout_nodeapi() to allow changes to workflow

I've just completed a site for which uc_node_checkout was used, but the order was only created when the node was in a specific workflow state, not on insert. It would be useful if uc_node_checkout_nodeapi() was refactored to (a) allow invocation of uc_node_checkout functions from external modules, and (b) wasn't hardcoded to create node-order associations only on node insert.