pgsql: Fix errors in key_column_usage.position_in_unique_constraint

Log Message:
-----------
Fix errors in key_column_usage.position_in_unique_constraint column recently
added to information_schema (per a SQL2003 addition). The original coding
failed if a referenced column participated in more than one pg_constraint
entry. Also, it did not work if an FK relied directly on a unique index
without any constraint syntactic sugar --- this case is outside the SQL spec,
but PG has always supported it, so it's reasonable for our information_schema
to handle it too. Per bug#2750 from Stephen Haberman.

Although this patch changes the initial catalog contents, I didn't force
initdb. Any beta3 testers who need the fix can install it via CREATE OR
REPLACE VIEW, so forcing them to initdb seems an unnecessary imposition.

Modified Files:
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pgsql/src/backend/catalog:
information_schema.sql (r1.37 -> r1.38)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/information_schema.sql.diff?r1=1.37&r2=1.38)