pgsql-server: Adjust cost_nonsequential_access() to have more reasonable

Log Message:
-----------
Adjust cost_nonsequential_access() to have more reasonable behavior
when random_page_cost has a small value. Per Manfred Koizar, though
I didn't use his equation exactly.

Modified Files:
--------------
pgsql-server/src/backend/optimizer/path:
costsize.c (r1.130 -> r1.131)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/optimizer/path/costsize.c.diff?r1=1.130&r2=1.131)