lib: allow listnode memory to be managed by the app

lib: allow listnode memory to be managed by the app

In most cases this memory is pre-allocated along with the base element.
Similarly it is stored in the base element to allow efficient del
without lookup (main reason for using DLL vs. SLL).

So (in most cases) there should be no need to manage the element/data
and listnode memories separately.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>