Account for SRFs in targetlists in planner rowcount estimates.

We made use of the ROWS estimate for set-returning functions used in FROM,
but not for those used in SELECT targetlists; which is a bit of an
oversight considering there are common usages that require the latter
approach.  Improve that.  (I had initially thought it might be worth
folding this into cost_qual_eval, but after investigation concluded that
that wouldn't be very helpful, so just do it separately.)  Per complaint
from David Johnston.

Back-patch to 9.2, but not further, for fear of destabilizing plan choices
in existing releases.

src/include/optimizer/clauses.h

@@ -55,6 +55,7 @@ extern bool contain_window_function(Node *clause);
 extern WindowFuncLists *find_window_functions(Node *clause, Index maxWinRef);
 
 extern double expression_returns_set_rows(Node *clause);
+extern double tlist_returns_set_rows(List *tlist);
 
 extern bool contain_subplans(Node *clause);
 

src/include/optimizer/planner.h

@@ -35,6 +35,9 @@ extern Plan *subquery_planner(PlannerGlobal *glob, Query *parse,
 				 bool hasRecursion, double tuple_fraction,
 				 PlannerInfo **subroot);
 
+extern void add_tlist_costs_to_plan(PlannerInfo *root, Plan *plan,
+									List *tlist);
+
 extern bool is_dummy_plan(Plan *plan);
 
 extern Expr *expression_planner(Expr *expr);