Fix things so that you can still do "select foo()" where foo is a SQL

Fix things so that you can still do "select foo()" where foo is a SQL
function returning setof record.  This used to work, more or less
accidentally, but I had broken it while extending the code to allow
materialize-mode functions to be called in select lists.  Add a regression
test case so it doesn't get broken again.  Per gripe from Greg Davidson.