Re-implement psql's FETCH_COUNT feature atop libpq's chunked mode.

Formerly this was done with a cursor, which is problematic since
not all result-set-returning query types can be put into a cursor.
The new implementation is better integrated into other psql
features, too.

Daniel Vérité, reviewed by Laurenz Albe and myself (and whacked
around a bit by me, so any remaining bugs are my fault)

Discussion: https://postgr.es/m/CAKZiRmxsVTkO928CM+-ADvsMyePmU3L9DQCa9NwqjvLPcEe5QA@mail.gmail.com

Author: tglsfdc