Fix PQsetdbLogin() backward compatibility problem.

If pghost == "" and pgport == "" then PQsetdbLogin() fails with a
error message:

 Is the postmaster running locally
        and accepting connections on Unix socket '/tmp/.s.PGSQL.0'?

I see many applications such as PHP fails due to this behavior.
Now if pgport == "", then it is assumed to be a DEF_PGPORT_STR. This
is the same behavior as the version prior 7.1.