Second try at fixing join alias variables. Instead of attaching miscellaneous

Second try at fixing join alias variables.  Instead of attaching miscellaneous
lists to join RTEs, attach a list of Vars and COALESCE expressions that will
replace the join's alias variables during planning.  This simplifies
flatten_join_alias_vars while still making it easy to fix up varno references
when transforming the query tree.  Add regression test cases for interactions
of subqueries with outer joins.