Change set_plan_references and join_references to take an rtable List

Change set_plan_references and join_references to take an rtable List
rather than a Query node; this allows set_plan_references to recurse
into subplans correctly.  Fixes core dump on full outer joins in
subplans.  Also, invoke preprocess_expression on function RTEs'
function expressions.  This seems to fix the planner's problems with
outer-level Vars in function RTEs.