[RFC] Add an until-0 loop in psql

Hi

When running database migrations with .sql files on a live database, it's not 
uncommon to have to run a migration in a loop to prevent a big lock on a 
table.
For instance if one want to delete some old datas from a big table one would 
write :

DELETE FROM big_table WHERE id IN (SELECT id FROM big_table WHERE bad = true 
LIMIT 1000);
VACUUM big_table;

Right now, doing this is quite inefficient. We either have to write a script 
in another language, or run psql in a shell loop and wait for the migration to 
stop altering rows.

The attached **proof of concept** patch (I insist, it's a 15 minutes hack 
sprint with no previous knowledge of psql code) implements an 'until-0' loop 
in psql.
The previous migration could be simply written as :

\until-0
BEGIN;
DELETE FROM big_table WHERE id IN (SELECT id FROM big_table WHERE bad = true 
LIMIT 1000);
VACUUM big_table;
COMMIT;
\end-until

And psql will execute it until there is no row affected in the inner queries.

I am willing to write a proper patch for this (I hope the tell/seek is an 
acceptable implementation…), but I prefer having some feedback first.

Thanks

 Pierre