pgsql: Improve outer-join-deduction logic to be able to propagate

Log Message:
-----------
Improve outer-join-deduction logic to be able to propagate equalities
through multiple join clauses.

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
pathkeys.c (r1.69 -> r1.70)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/pathkeys.c.diff?r1=1.69&r2=1.70)