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

libc/include/search.h

@@ -29,6 +29,9 @@ typedef struct node {
 
 __BEGIN_DECLS
 
+void insque(void*, void*);
+void remque(void*);
+
 void* lfind(const void*, const void*, size_t*, size_t, int (*)(const void*, const void*));
 void* lsearch(const void*, void*, size_t*, size_t, int (*)(const void*, const void*));