pgsql: Fix "constraint_exclusion = partition" logic so that it will also

Log Message:
-----------
Fix "constraint_exclusion = partition" logic so that it will also attempt
constraint exclusion on an inheritance set that is the target of an UPDATE
or DELETE query. Per gripe from Marc Cousin. Back-patch to 8.4 where
the feature was introduced.

Modified Files:
--------------
pgsql/src/backend/nodes:
outfuncs.c (r1.384 -> r1.385)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c?r1=1.384&r2=1.385)
pgsql/src/backend/optimizer/plan:
planner.c (r1.266 -> r1.267)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c?r1=1.266&r2=1.267)
pgsql/src/backend/optimizer/util:
plancat.c (r1.162 -> r1.163)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/plancat.c?r1=1.162&r2=1.163)
pgsql/src/include/nodes:
relation.h (r1.185 -> r1.186)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h?r1=1.185&r2=1.186)