Set all variable-length fields of pg_attribute to null on column drop

Set all variable-length fields of pg_attribute to null on column drop

When a column is dropped, the fields attacl, attoptions, and
attfdwoptions were kept unchanged.  This is probably harmless, but it
seems wasteful, and leaves potentially dangling data lying around (for
example, attacl could contain references to users that are later also
dropped).

Change this to set those fields to null when a column is marked as
dropped.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/249d819d-1763-4580-8110-0bf91a0f08b7@eisentraut.org