receiver/announced: Ensure multicast interface is set (#104)
receiver/announced: Ensure multicast interface is set (#104)

It seems that using multicast on addresses with an interface specification
requires setting the multicast interface beforehand. Else no message
is transmitted. Tested with NodeJS v10.15.2.
hopglass/hopglass-server,plumpudding/hopglass-server,freifunk-saar/hopglass-server,eberhab/hopglass-server