Remove maybe_set_name

Remove maybe_set_name

Outside of unit tests, the name parameter in dns_qpiter_<...> and
dns_qpchain_<...> is only used in context where the name can be
extracted directly from the underlying node.

This commits modifies the signatures of dns_qpiter_<...> and
dns_qpchain_<...> not to have a name parameter. Where the name parameter
was needed, we now query the node and copy the name directly from it.

This allows us to remove maybe_set_name from qp.c. Besides simplifying
the API, this leads to a performance speedup for NXDOMAIN handling,
as we avoid calling maybe_set_name inside step, and maybe_set_name is
very inefficient.

A copy of the implementation maybe_set_name is retained for the unit
tests.