patch for PGAPI_Columns with Show system tables turned on

Hello,

  when I take a look at VB6 and RowCount (again) and I turn Parse
Statement option on there is problem with SQLColAttribute for
select * from pg_class. I found that pg_* is forbidden in PGAPI_Columns
when Show system tables is turned off. I think it's not right behaviour.

I write small patch which changes this behaviour. When is pg_* asked
in PGAPI_Columns directly it doesn't matter if Show system tables is
turned on or off.

Please review and comment

Luf