restrict postcode searches to postcode in first token

restrict postcode searches to postcode in first token

In structured queries we should only assume that it is
a postcode search when only the postcode and optionally
the country is given. If any other term is present, it
is better to avoid the search for postcode as it yields
too many bad searches. Given that the terms in a structured
query are ordered, this means that the postcode must be
the first token just like in the unstructured query.

Fixes #1988.