the 'predecessor' argument to dns_qp_lookup() turns out not to
be sufficient for our needs: the predecessor node in a QP database
could have become "empty" (for the current version) because of an
update or because cache data expired, and in that case the caller
would have to iterate more than one step back to find the predecessor
node that it needs.

it may also be necessary for a caller to iterate forward, in
order to determine whether a node has any children.

for both of these reasons, we now replace the 'predecessor'
argument with an 'iter' argument. if set, this points to memory
with enough space for a dns_qpiter object.

when an exact match is found by the lookup, the iterator will be
pointing to the matching node. if not, it will be pointing to the
lexical predecessor of the nae that was searched for.

a dns_qpiter_current() method has been added for examining
the current value of the iterator without moving it in either
direction.