We don't enforce NO SCROLL cursor restrictions

We don't actually prevent you from scrolling a NO SCROLL cursor:

regression=# begin;
BEGIN
regression=*# declare c no scroll cursor for select * from int8_tbl;
DECLARE CURSOR
regression=*# fetch all from c;
q1 | q2
------------------+-------------------
123 | 456
123 | 4567890123456789
4567890123456789 | 123
4567890123456789 | 4567890123456789
4567890123456789 | -4567890123456789
(5 rows)

regression=*# fetch absolute 2 from c;
q1 | q2
-----+------------------
123 | 4567890123456789
(1 row)

There are probably specific cases where you do get an error,
but we don't have a blanket you-can't-do-that check. Should we?

The reason this came to mind is that while poking at [1]
I noticed that commit ba2c6d6ce has created some user-visible
anomalies for non-scrollable cursors WITH HOLD. If you advance
the cursor a bit, commit, and then try to scroll the cursor,
it will work but the part of the output that you advanced over
will be missing. I think we should probably throw an error
to prevent that from being visible. I'm worried though that
putting in a generic prohibition may break applications that
used to get away with this kind of thing.

regards, tom lane

[1] https://www.postgresql.org/message-id/CAPV2KRjd%3DErgVGbvO2Ty20tKTEZZr6cYsYLxgN_W3eAo9pf5sw%40mail.gmail.com