selftests: forwarding: add helpers for IP multicast group joins/leaves

Extend the forwarding library with calls to some small C programs which
join an IP multicast group and send some packets to it. Both IPv4 and
IPv6 groups are supported. Use cases range from testing IGMP/MLD
snooping, to RX filtering, to multicast routing.

Testing multicast traffic using msend/mreceive is intended to be done
using tcpdump.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>