Further review for re-implementation of psql's FETCH_COUNT feature.

Further review for re-implementation of psql's FETCH_COUNT feature.

Alexander Lakhin noted an obsolete comment, which led me to revisit
some other important comments in the patch, and that study turned up a
couple of unintended ways in which the chunked-fetch code path didn't
match the normal code path in ExecQueryAndProcessResults.  The only
nontrivial problem is that it didn't call PrintQueryStatus, so that
we'd not print the final status result from DML ... RETURNING
commands.  To avoid code duplication, move the filter for whether a
result is from RETURNING from PrintQueryResult to PrintQueryStatus.

Discussion: https://postgr.es/m/0023bea5-79c0-476e-96c8-dad599cc3ad8@gmail.com