psql's FETCH_COUNT (cursor) is not being respected for CTEs

Hi -hackers,

I've spent some time fighting against "out of memory" errors coming
out of psql when trying to use the cursor via FETCH_COUNT. It might be
a not so well known fact (?) that CTEs are not executed with cursor
when asked to do so, but instead silently executed with potential huge
memory allocation going on. Patch is attached. My one doubt is that
not every statement starting with "WITH" is WITH(..) SELECT of course.

Demo (one might also get the "out of memory for query result"):

postgres(at)hive:~$ psql -Ant --variable='FETCH_COUNT=100' -c "WITH data
AS (SELECT generate_series(1, 20000000) as Total) select repeat('a',
100) || data.Total || repeat('b', 800) as total_pat from data;"
Killed
postgres(at)hive:~$ tail -4 /var/log/postgresql/postgresql-14-main.log
[..]
2023-01-04 12:46:20.193 CET [32936] postgres(at)postgres LOG: could not
send data to client: Broken pipe
[..]
2023-01-04 12:46:20.195 CET [32936] postgres(at)postgres FATAL:
connection to client lost

With the patch:
postgres(at)hive:~$ /tmp/psql16-with-patch -Ant
--variable='FETCH_COUNT=100' -c "WITH data AS (SELECT
generate_series(1, 20000000) as Total) select repeat('a', 100) ||
data.Total || repeat('b', 800) as total_pat from data;" | wc -l
20000000
postgres(at)hive:~$

Regards,
-Jakub Wartak.