groupcast_demo
   A demo showing how unicast/anycast/multicast/broadcast can be sent
   loss free from a client to multiple servers, demonstrating the group
   communication feature's flow control and sequence guarantee.
   ../test/group_test is is a more elaborated version of this program.

Usage:
$group_cast [-b] [-m] [-a] [-l <msglen>]
            [-b send group broadcast (default off)
            [-m send group multicast (default off)
            [-a send group anycast (default off)
            [-r request response on sent Xcast (default off)
            [-d destination member instance (default 0)
            [-i member instance (default 0)
            [-l <msglen>] message size (default 66000)
            [-L loopback broadcast/multicast/anycast (default off)
            -a, -b , -m are mutually exclusive
