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

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/commands:
explain.c (r1.146 -> r1.147)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.146&r2=1.147)
pgsql/src/backend/utils/adt:
ruleutils.c (r1.218 -> r1.219)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.218&r2=1.219)
pgsql/src/include/utils:
builtins.h (r1.278 -> r1.279)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h.diff?r1=1.278&r2=1.279)