Repair bug reported by ldm@apartia.com: Append nodes, which don't

actually use their targetlist, are given a targetlist that is just a
pointer to the first appended plan's targetlist.  This is OK, but what
is not OK is that any sub-select expressions in said tlist were being
entered in the subPlan lists of both the Append and the first appended
plan.  That led to two startup and two shutdown calls for the same
plan node at exec time, which led to crashes.  Fix is to not generate
a list of subPlans for an Append node.  Same problem and fix apply
to other node types that don't have a real, functioning targetlist:
Material, Sort, Unique, Hash.