ALTER TABLE ... SET TABLESPACE pg_default

Greetings,

Harking back to 10 years ago when tablespaces were added, it looks
like we originally figured that users didn't need permissions to
create tables in the database default, per 2467394e. That strikes me
as perfectly fair. Unfortunately, the later addition of
ALTER TABLE ... SET TABLESPACE (af4de814) didn't get the memo about
the default tablespace being special in this regard and refuses to let
a user move their tables into the default tablespace, even though they
can do so via 'CREATE TABLE ... AS SELECT * FROM ...'.

Barring objections, I'll add the same conditional around the AclCheck
in ATPrepSetTableSpace() as exists in DefineRelation() to allow users
to ALTER TABLE ... SET TABLESPACE into the database's default
tablespace and backpatch accordingly.

Thanks,

Stephen