get predecessor name in dns_qp_findname_ancestor()

get predecessor name in dns_qp_findname_ancestor()

dns_qp_findname_ancestor() now takes an optional 'predecessor'
parameter, which if non-NULL is updated to contain the DNSSEC
predecessor of the name searched for. this is done by constructing
an iterator stack while carrying out the search, so it can be used
to step backward if needed.