plpgsql: Don't generate parallel plans for RETURN QUERY.

Commit 7aea8e4f2d allowed a parallel
plan to be generated when for a RETURN QUERY or RETURN QUERY EXECUTE
statement in a PL/pgsql block, but that's a bad idea because plplgsql
asks the executor for 50 rows at a time.  That means that we'll always
be running serially a plan that was intended for parallel execution,
which is not a good idea.  Fix by not requesting a parallel plan from
the outset.

Per discussion, back-patch to 9.6.  There is a slight risk that, due
to optimizer error, somebody could have a case where the parallel plan
executed serially is actually faster than the supposedly-best serial
plan, but the consensus seems to be that that's not sufficient
justification for leaving 9.6 unpatched.

Discussion: http://postgr.es/m/CA+TgmoZ_ZuH+auEeeWnmtorPsgc_SmP+XWbDsJ+cWvWBSjNwDQ@mail.gmail.com
Discussion: http://postgr.es/m/CA+TgmobXEhvHbJtWDuPZM9bVSLiTj-kShxQJ2uM5GPDze9fRYA@mail.gmail.com

src/pl/plpgsql/src/pl_exec.c

@@ -3023,7 +3023,7 @@ exec_stmt_return_query(PLpgSQL_execstate *estate,
 	if (stmt->query != NULL)
 	{
 		/* static query */
-		exec_run_select(estate, stmt->query, 0, &portal, true);
+		exec_run_select(estate, stmt->query, 0, &portal, false);
 	}
 	else
 	{
@@ -3031,7 +3031,7 @@ exec_stmt_return_query(PLpgSQL_execstate *estate,
 		Assert(stmt->dynquery != NULL);
 		portal = exec_dynquery_with_params(estate, stmt->dynquery,
 										   stmt->params, NULL,
-										   CURSOR_OPT_PARALLEL_OK);
+										   0);
 	}
 
 	/* Use eval_mcontext for tuple conversion work */