add dlist interface stubs for implementation of:
- dlist_init
- dlist_new_list
- dlist_new_node
- dlist_print_node
- dlist_delete_node
- dlist_delete_list
- dlist_push_node
- dlist_append_node
- dlist_pop_node
- dlist_find_node
- dlist_find_index_of
- dlist_remove_node
- dlist_remove_node_at
- dlist_foreach_node
- dlist_delete_all_nodes
- dlist_reverse_list
- dlist_get_size
- dlist_push_list
- dlist_append_list
- dlist_split_list
- dlist_split_list_at