Implement twalk(3), add unit tests.

I've also added insque(3) and remque(3) (from NetBSD because the OpenBSD
ones are currently broken for non-circular lists).

I've not added the three hash table functions that should be in this header
because they operate on a single global hash table and thus aren't likely
to be useful.

Bug: https://code.google.com/p/android/issues/detail?id=73719
Change-Id: I97397a7b921e2e860fd9c8032cafd9097380498a
(cherry picked from commit 3e424d0a241f8ae5194514dabc4ad899f5a5416d)