Initialize tableoid field correctly when dumping foreign data wrappers and

Initialize tableoid field correctly when dumping foreign data wrappers and
servers. AFAICT it's harmless at the moment because nothing can depend on
either, but as soon as we introduce an object type with such dependencies,
tableoid needs to be set or pg_dump will fail to interpret the dependencies
correctly. In theory, I guess the uninitialized garbage in tableoid could
cause the object to be mistaken for some other object with same OID as well.