Although the insert() function allows new nodes to be inserted at the head
and the tail of a list, the addhead() and addtail() functions will do so
with higher efficiency.  Adding to the head or tail of a list is common
practice in first-in-first-out (fifo) or last-in-first-out (lifo or stack)
operations.  For example, AddHead(header,node) would insert the node at
the head of the specified list.