Auto merge of #1555 - flocknetworks:ip_mreqn, r=gnzlbg

Add struct ip_mreqn: ip multicast req with intf id

The ip_mreqn struct has an additional interface id parameter. This
allows programming IPv4 multicast groups using the interface id
alongside the src IPv4 address.
Linux has supported the ip_mreqn struct since Linux 2.2
ref: man ip.7