Fix EXPLAIN so that it can drill down through multiple levels of subplan

Fix EXPLAIN so that it can drill down through multiple levels of subplan
when trying to locate the referent of a RECORD variable.  This fixes the
'record type has not been registered' failure reported by Stefan
Kaltenbrunner about a month ago.  A side effect of the way I chose to
fix it is that most variable references in join conditions will now be
properly labeled with the variable's source table name, instead of the
not-too-helpful 'outer' or 'inner' we used to use.