Fix optimizer to not try to push WHERE clauses down into a sub-SELECT that

Fix optimizer to not try to push WHERE clauses down into a sub-SELECT that
has a DISTINCT ON clause, per bug report from Anthony Wood.  While at it,
improve the DISTINCT-ON-clause recognizer routine to not be fooled by out-
of-order DISTINCT lists.
Also, back-patch earlier fix to not push down into sub-SELECT with LIMIT.