Also adjust expected regression results for commit ed7b80d78ff89db0d3dac6124e9cb2288c786c45
Add support for modify actions based on Informix ROWID.

Formerly, the Informix FDW performed modify actions (DML) commands
with an updatable cursor. It turned out that this didn't work out very
well in cases, where an UPDATE...FROM or DELETE...USING SQL statement
was used. If the optimizer has a plan choosen, which was based on a Hash Join,
where the remote table was hashed first, the cursor can't be reused, since it
was already positioned at the end of the ForeignScan, which will error out, or
in bad cases, produce wrong results.

UPDATE inttest SET f1 = t.id
FROM local_inttest t
WHERE t.id = f1 AND t.id BETWEEN 1 AND 2000;

If a hash join is involved where the tuples from the ForeignScan are hashed
first, the cursor needs to be repositioned at the right tuple again, when the
ForeignUpdate occurs. Since we can't easily reposition the cursor, we just error
out.

Switch the default behavior to ROWID usage instead, which will perform any
modify action like UPDATE with a ROWID formerly retrieved as a resjunk
column during the ForeignScan. This terminates the error condition above and
makes DML actions robust against any plan the optimizer wants to choose.
So instead of UPDATE ... remote_table WHERE CURRENT OF the FDW now employs
any UPDATE or DELETE statement with WHERE rowid = <ROWID>. However, there might
be cases, where the rowid can't be used atm: Fragmented tables in Informix don't
include the ROWID per default, since in such organized tables the ROWID is not
unique across all fragments. You'll need to create the fragmented tables WITH ROWIDs
to use the FDW with its default behavior. It is possible to switch back to updatable
cursors by setting the option "disable_rowid" for a foreign table, e.g.

ALTER FOREIGN TABLE inttest OPTIONS(ADD disable_rowid '1');

"disable_rowid" just needs to be set, to enable ROWIDs again, the options just
needs to be dropped.

The ROWID is encoded as an ItemPointer when transported through the executor.

This commit isn't tested against older PostgreSQL releases yet, further commits will
adjust things if necessary.