Provide backward-compatible unordered_map/set

Provide backward-compatible unordered_map/set

We were defining our own stap_map with a ::type to let us use typedefs
to use the new unordered_map if available, or hash_map otherwise.  Since
unordered_map is the future direction, I'm changing our code to use that
directly.  The backward-compatible version is a #define to hash_map,
which has a compatible interface.

While I'm at it, let's also define unordered_multimap, unordered_set,
and unordered_multiset.

* unordered.h: New.
* dwflpp.h (stap_map): Removed.
  (cache typedefs): Use the unordered name now.