Centralize some ALTER <whatever> .. SET SCHEMA checks.

Centralize some ALTER <whatever> .. SET SCHEMA checks.

Any flavor of ALTER <whatever> .. SET SCHEMA fails if (1) the object
is already in the new schema, (2) either the old or new schema is
a temp schema, or (3) either the old or new schema is the TOAST schema.

Extraced from a patch by Dimitri Fontaine, with additional hacking by me.