pg_dump --clean w/ <= 7.2 server

When running pg_dump --clean against a server that doesn't have schemas 
the namespace is blank and ends up producing a dump full off things like:

DROP TABLE "".tab;

The attached patch only includes a schema if one exists.  There are
numerous comments about the DROPs needing to be fully qualified to avoid
conflicting with pg_catalog, but this should be mostly safe because it
would require a user defined object to be present in a later server
version with the same name, and it is difficult to impossible to drop
system objects anyway.

Kris Jurka