pgsql: Fix information_schema.key_column_usage to show correct value of

Log Message:
-----------
Fix information_schema.key_column_usage to show correct value of
position_in_unique_constraint (column newly added per SQL2003).
Greg Mullane

Modified Files:
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pgsql/src/backend/catalog:
information_schema.sql (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/information_schema.sql.diff?r1=1.34&r2=1.35)