Code review for pushing LIMIT through subqueries.

Code review for pushing LIMIT through subqueries.

Minor improvements for commit 1f6d515a6.  We do not need the (rather
expensive) test for SRFs in the targetlist, because since v10 any
such SRFs would appear in separate ProjectSet nodes.  Also, make the
code look more like the existing cases by turning it into a simple
recursion --- the argument that there might be some performance
benefit to contorting the code seems unfounded to me, especially since
any good compiler should turn the tail-recursion into iteration anyway.

Discussion: http://postgr.es/m/CADE5jYLuugnEEUsyW6Q_4mZFYTxHxaVCQmGAsF0yiY8ZDggi-w@mail.gmail.com