Further split single ds creation into two, sigh

Further split single ds creation into two, sigh

- Allow rpmdsCurrent() to share the pool and id's of its "parent" ds
  instead of having to repeatedly create and tear down entire pools
  just for a couple of strings. Used by python bindings for rpmds
  iteration so we'll want to be reasonably efficient.
- For now, rpmdsSingle() and rpmdsThis() always get a private pool,
  wasteful as it might be, but at least now we can freeze them.