psql's is_select_command is naive

It knows that queries beginning with "select" or "values" are select
commands, but it seems not to be clued in about "table" and "with".

For now, we can probably get by with just adding those to the list of
things it checks for.  But I wonder what we'll do about this if we
ever get CTEs for insert/update/delete statements...  you'd have to
parse the whole darn thing to figure out whether it was WITH...SELECT
or WITH...INSERT or WITH...INSERT...RETURNING.

-- 
Robert Haas
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