pgsql: Change Agg and Group nodes so that Vars contained in their

Log Message:
-----------
Change Agg and Group nodes so that Vars contained in their targetlists
and quals have varno OUTER, rather than zero, to indicate a reference to
an output of their lefttree subplan.  This is consistent with the way
that every other upper-level node type does it, and allows some simplifications
in setrefs.c and EXPLAIN.

Modified Files:
--------------
    pgsql/src/backend/commands:
        explain.c (r1.157 -> r1.158)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.157&r2=1.158)
    pgsql/src/backend/executor:
        nodeAgg.c (r1.150 -> r1.151)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeAgg.c.diff?r1=1.150&r2=1.151)
        nodeGroup.c (r1.68 -> r1.69)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeGroup.c.diff?r1=1.68&r2=1.69)
        nodeResult.c (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeResult.c.diff?r1=1.39&r2=1.40)
    pgsql/src/backend/optimizer/plan:
        setrefs.c (r1.131 -> r1.132)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/setrefs.c.diff?r1=1.131&r2=1.132)
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.249 -> r1.250)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.249&r2=1.250)