wdp: Change functions that use wdp_port to avoid awkward memory semantics.

wdp: Change functions that use wdp_port to avoid awkward memory semantics.

wdp_port_query_by_number(), wdp_port_query_by_name(), and
wdp_port_list() all had semantics for the data that they returned that
were difficult to describe and easy to get wrong.  This commit changes them
to be easier to understand and to implement.

Suggested by partner.