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