Set Socket interface using interface index

In C, one can set a socket's interface using setsockopt(socket, SOL_IP, IP_MULTICAST_IF, &mreqn, sizeof(mreqn)) where mreqn is

struct ip_mreqn {
    struct in_addr imr_multiaddr; /* IP multicast group address */
    struct in_addr imr_address;   /* IP address of local interface */
    int            imr_ifindex;   /* interface index */
};

sockets2 offers the set_multicast_if_v4 function, but only accepts an address as parameter. I would like to set the interface using sockets2 based on an interface index.

Start by reading sockets2's set_multicast_if_v4 function and compare its address-based behavior with the C setsockopt call using ip_mreqn shown in the issue. Done means sockets2 provides a way to select the IPv4 multicast interface by index, with behavior covered by the relevant tests.