PQmakeResult patch

Here is a patch to add a few functions to libpq:

PQmakeResult: creates a new result with attributes but no rows
PQsetValue: set a field inside a result, creating a row if necessary
(but only one at a time)
PQresultAlloc (basically public wrapper to existing internal function).

also, the attribute structure (but not the result) is moved to the
public header.

We thought these functions, particularly PQsetValue, are something
positive that came out of the libpqtypes dicussions.

merlin