Impl. createDLList(), freeDLList(), freeDLNode().

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Implement and document `createDLList()`, `freeDLList()`, and
	`freeDLNode()`. The first two were renamed from `createSLList()`
	and `freeSLList()`, as they were initially incorrectly declared.