No leaks so far with server execution.

* Newly created dnames have refcount=1, caller is responsible for releasing them.
* Copied dnames have also refcount=1 (as they are new).

To do:
* Carefully implement more uses of dname not protected by refcounting.
* Consider renaming dnslib_dname_copy() do _duplicate() as it creates duplicate.
* Consider shallow copying of dname, as it's possible with refcounting.
* Test and review.

Commit refs #993.