Re-allow planner to use Merge Append to efficiently implement UNION.

This reverts commit 7204f35919,
thus restoring 66c0185a3 (Allow planner to use Merge Append to
efficiently implement UNION) as well as the follow-on commits
d5d2205c8, 3b1a7eb28, 7487044d6.

Per further discussion on pgsql-release, we wish to ship beta1 with
this feature, and patch the bug that was found just before wrap,
rather than shipping beta1 with the feature reverted.

src/include/nodes/pathnodes.h

@@ -400,6 +400,8 @@ struct PlannerInfo
 	List	   *distinct_pathkeys;
 	/* sortClause pathkeys, if any */
 	List	   *sort_pathkeys;
+	/* set operator pathkeys, if any */
+	List	   *setop_pathkeys;
 
 	/* Canonicalised partition schemes used in the query. */
 	List	   *part_schemes pg_node_attr(read_write_ignore);