mentioned in FROM but not elsewhere in the query: such tables should be
joined over anyway.  Aside from being more standards-compliant, this allows
removal of some very ugly hacks for COUNT(*) processing.  Also, allow
HAVING clause without aggregate functions, since SQL does.  Clean up
CREATE RULE statement-list syntax the same way Bruce just fixed the
main stmtmulti production.
CAUTION: addition of a field to RangeTblEntry nodes breaks stored rules;
you will have to initdb if you have any rules.