Test IPv4 multicast packets.

Use two programs to send and receive multicast packets.  After
setting up the socket, the receiver forks and execs the sender to
avoid races.  If the test fails, the receiver runs into a timeout.

The test programs are:
mcsend  - send one multicat UDP packet
mcrecv  - receive one multicast UDP packet
mcroute - route one multicast UDP packet

The options for mcsend and mcrecv and mcroute are:
-b              fork to background after setup
-f file         print message to log file, default stdout
-g group        multicast group, default 224.0.0.123
-i ifaddr       multicast interface address
-l loop         disable or enable loopback, 0 or 1
-m message      message in payload, maximum 255 characters, default foo
-n timeout      expect not to receive any message until timeout
-p port         destination port number, default 12345
-o outaddr      outgoing interface address
-r timeout      receive timeout in seconds
-t ttl          set multicast ttl
mcsend ...      after setting up receive, fork and exec send command

With mcroute packets are sent over a multicast router.  The kernel
route is installed statically.  The machines sender, router, receiver
are involved.  Receiver is on the local machine, route is on remote
machine.  The sender can share the local machine or be started on
a target machine, depending on the setup.  This is controlled via
environment.