BUG #7873: pg_restore --clean tries to drop tables that don't exist

The following bug has been logged on the website:

Bug reference:      7873
Logged by:          Dave Rolsky
Email address:      autarch@urth.org
PostgreSQL version: 9.2.3
Operating system:   Linux
Description:        

When you pass the --clean option to pg_restore it tries to drop tables
without checking if they exist. This results in lots of error output. If
you're running pg_restore via an automated process it's very hard to
distinguish between these "ok" errors and real errors.

It should be using "DROP TABLE IF EXISTS" and the equivalent for
constraints.