pgsql: Quick hack to allow the outer query's tuple_fraction to be passed

Log Message:
-----------
Quick hack to allow the outer query's tuple_fraction to be passed down
to a subquery if the outer query is simple enough that the LIMIT can
be reflected directly to the subquery. This didn't use to be very
interesting, because a subquery that couldn't have been flattened into
the upper query was usually not going to be very responsive to
tuple_fraction anyway. But with new code that allows UNION ALL subqueries
to pay attention to tuple_fraction, this is useful to do. In particular
this lets the optimization occur when the UNION ALL is directly inside
a view.

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
allpaths.c (r1.133 -> r1.134)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c.diff?r1=1.133&r2=1.134)
pgsql/src/backend/optimizer/plan:
planmain.c (r1.84 -> r1.85)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planmain.c.diff?r1=1.84&r2=1.85)
pgsql/src/include/nodes:
relation.h (r1.113 -> r1.114)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h.diff?r1=1.113&r2=1.114)