pgsql: Re-implement psql's FETCH_COUNT feature atop libpq's chunked mod

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: /messages/by-id/CAKZiRmxsVTkO928CM+-ADvsMyePmU3L9DQCa9NwqjvLPcEe5QA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/90f5178211cd63ac16fb8c8b2fe43d53d2854da1

Modified Files
--------------
src/bin/psql/common.c | 522 ++++++++++++-------------------------
src/bin/psql/t/001_basic.pl | 6 +-
src/test/regress/expected/psql.out | 4 +-
src/test/regress/sql/psql.sql | 4 +-
4 files changed, 171 insertions(+), 365 deletions(-)