tests/netinet: extend IPv4 multicast testing framework

Provide a helper program multicast-receive.c that joins arbitrary group on
arbitrary specified interface and receives a single datagram and prints it
out.

In multicast.sh provide a template with two jails connected by two
interface pairs, so that we can check sends/receives in presence of
multiple interfaces.

Compose a few basic tests to check legacy and new IP_ADD_MEMBERSHIP and
MCAST_JOIN_GROUP.