Improved the robustness against funny firewall configurations. PGT uses two
connections: A command connection on port 1536, and a data connection in the
port range between 31000 and 31999 (inclusive). If the connection on port
1536 is allowed, but the data-connection is blocked by the firewall, PGT
would not fall back to HTTP (port 80 only), but simply fail. This has now been
fixed.