Probe: support pinging multiple multicast addresses

probe.sh -addr 224.0.75.75 pings all members in a network which joined multicast address 224.0.75.75.
However, sometimes, it would be desirable to ping multiple multicast groups.
This can now be done by specifying -addr multiple times, e.g.
probe.sh -addr 224.0.75.75 -addr 228.5.5.5
Also, IPv6 and IPv4 clusters can be pinged at the same time, e.g.
java org.jgroups.tests.Probe -addr ff0e::0:75:75 -addr 224.0.75.75
We can also list multicast and unicast addresses:
java org.jgroups.tests.Probe -addr ff0e::0:75:75 -addr 192.168.1.3