Finish porting hammer's library to windows

We port registry by importing the (public domain) openbsd implementation
of the tfind/tsearch POSIX binary tree search functions.

These are only necessary when building on non-posix platforms

src/search.h

@@ -0,0 +1,15 @@
+#if defined(_MSC_VER)
+/* find or insert datum into search tree */
+void *tsearch(const void *vkey, void **vrootp,
+              int (*compar)(const void *, const void *));
+
+/* delete node with given key */
+void * tdelete(const void *vkey, void **vrootp,
+               int (*compar)(const void *, const void *));
+
+/* Walk the nodes of a tree */
+void twalk(const void *vroot, void (*action)(const void *, VISIT, int));
+
+#else
+#include <search.h>
+#endif