Remove hack in pg_tablespace_aclmask() that disallowed permissions

Remove hack in pg_tablespace_aclmask() that disallowed permissions
on pg_global even to superusers, and replace it with checks in various
other places to complain about invalid uses of pg_global.  This ends
up being a bit more code but it allows a more specific error message
to be given, and it un-breaks pg_tablespace_size() on pg_global.
Per discussion.