Fix ALTER INDEX RENAME so that if the index belongs to a unique or primary key

Fix ALTER INDEX RENAME so that if the index belongs to a unique or primary key
constraint, the constraint is renamed as well.  This avoids inconsistent
situations that could confuse pg_dump (not to mention humans).  We might at
some point provide ALTER TABLE RENAME CONSTRAINT as a more general solution,
but there seems no reason not to allow doing it this way too.  Per bug #3854
and related discussions.