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

src/test/regress/sql/psql.sql

@@ -1161,14 +1161,14 @@ SELECT 4 AS \gdesc
 \echo 'last error message:' :LAST_ERROR_MESSAGE
 \echo 'last error code:' :LAST_ERROR_SQLSTATE
 
--- check row count for a cursor-fetched query
+-- check row count for a query with chunked results
 \set FETCH_COUNT 10
 select unique2 from tenk1 order by unique2 limit 19;
 \echo 'error:' :ERROR
 \echo 'error code:' :SQLSTATE
 \echo 'number of rows:' :ROW_COUNT
 
--- cursor-fetched query with an error after the first group
+-- chunked results with an error after the first chunk
 select 1/(15-unique2) from tenk1 order by unique2 limit 19;
 \echo 'error:' :ERROR
 \echo 'error code:' :SQLSTATE