dgram: added setMulticastInterface()

Add wrapper for uv's uv_udp_set_multicast_interface which provides the
sender side mechanism to explicitly select an interface. The
equivalent receiver side mechanism is the optional 2nd argument of
addMembership().

PR-URL: https://github.com/nodejs/node/pull/7855


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>