Add a Node to a List.
Add a Node to a List. The Node structure passed in is either just a node with space for pointers to previous and next node, and some data, or it can be the head of a larger structure. It will be inserted at the bottom of the List, as element list->tail.
Each time an item is added to a List, its size element is increased by one. This can be used to instantly tell how many items are in the list.