Tune outgoing/incoming interface configuration

According to http://www.tldp.org/HOWTO/Multicast-HOWTO-6.html:

"If the host has more than one interface and the IP_MULTICAST_IF option is not
set, multicast transmissions are sent from the default interface, although the
remaining interfaces might be used for multicast forwarding if the host is
acting as a multicast router.".

Thus make sure self.intf can send and receive
multicast as it might not be the system's default interface. At least on OS X
10.9.5 this isn't as straightforward as it sounds, thus make sure it does no harm.