pg_dump should use current_database() instead of PQdb()

A report from a pgbouncer user revealed that running pg_dump -C/--create
does not work through a connection proxy if the virtual database name on
the proxy does not match the real database name on the database server.
That's because pg_dump looks up the database to be dumped using the
information from PQdb().  It should be using current_database() instead.
 (The code was quite likely written before current_database() was
available (PG 7.3)).

See attached patch.

There are a few other uses of PQdb() in pg_dump, but I think those are
OK because they relate to connection information.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services