Disallow NULLS NOT DISTINCT indexes for primary keys

A unique index which is created with non-distinct NULLS cannot be
used for backing a primary key constraint.  Make sure to disallow
such table alterations and teach pg_dump to drop the non-distinct
NULLS clause on indexes where this has been set.

Bug: 17720
Reported-by: Reiner Peterke <zedaardv@drizzle.com>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/17720-dab8ee0fa85d316d@postgresql.org
