More npgsql compatibility in the PG server

* SET TRANSACTION ISOLATION LEVEL. Gives a warning (once per connection) if not requesting SERIALIZABLE, but not an error.
* SHOW TRANSACTION ISOLATION LEVEL will return serializable, not what you asked for.
* SET lc_monetary='C';
* unlisten *. I hope that this is done in a way that does not introduce new garbage into the parser and hopefully without any particular overhead for regular statements, since it must get a chance before the parser is called.