Doubly linked lists unit test and update to implementation.

Update the doubly linked list implementation.  Now safe traversing can
insert before and after the current node when traversing in either
direction.

Updated the linked lists unit test test_linkedlist to also test doubly
linked lists.  The old test_dlinkedlist requires a manual check of results
and probably should be removed.

Review: https://reviewboard.asterisk.org/r/1569/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@347110 65c4cc65-6c06-0410-ace0-fbb531ad65f3