Allow -z option to work in server hub mode. Enable DNS lookups when connecting as a hub client.

Added IPv6 support. (Not tested on Win32 yet, but necessary OS specific changes have been made.) The trick of binding two sockets to the same port by shutting down write capabilities on one of them didn't work as expected on IPv6. So instead, a single full duplex socket is shared by two half duplex QUdpSocket objects.