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)