WIP: Initial attempt at adding loopback support

This is an attempt to solve issue #28.

It basically duplicates the code from glib in the g_socket_listener_add_inet_port, for one that takes an additional boolean saying whether the address should be loopback or not.

This doesn't exactly duplicate the method, because that one accessed private members of a structure and emitted signals using an object that didn't appear exported. It also drops support for source_object parameter because we don't appear to use this, and the method doesn't have to be ultra generic.

I also don't know enough about settings or coding for glib to ensure that when the loopback setting is changed, the service rebinds/restarts so that is honours the loopback setting.

As this is my first time submitting more than a 2-line patch for C/gnome code, I'm more than happy for suggested fixes or any other improvements that can be made. 5:)