Packets delivery in IPv4/IPv6 multicast is handled together by
multicast-routing daemon and by kernel. The MR daemon installs routes
into kernel, and the kernel actually routes the traffic.

We want the overall solution to obey to the following properties:

	every member of the group

	* can send a packet to the group via single uniform send call.
	* That packet will be delivered to all members of the group -
	  both on the same and on other machines, exactly once to each recipient.

This suggests sending/reception to be organized via one virtual
interface and then forwarded outside/into-inside as needed.

This patch adds topo-ABCDloop which creates a 4-nodes network with
cycle. It then manually installs static multicast routes the way, with
avoiding loops and that a packet sent from particular sender is
delivered to all other members of the network exactly once.

The sending/reception is organized via mcast-rx/mcast-tx veth pair, or
via loopback if the kernel has the following fixes

	kirr/linux@25bcc760
	kirr/linux@43eadf90

It was tested to work ok via

	# on A
	# ./tmcast.py txrx6 A
	joining ff1e::1 @ [7]mcast-rx
	tx: A.1 ...
	rx: A.1
	tx: A.2 ...
	rx: A.2
	tx: A.3 ...
	rx: A.3

	# on B
	# ./tmcast.py rx6
	joining ff1e::1 @ [7]mcast-rx
	rx: A.1
	rx: A.2
	rx: A.3

	# on C
	# ./tmcast.py rx6
	joining ff1e::1 @ [7]mcast-rx
	rx: A.1
	rx: A.2
	rx: A.3

	# on D
	# ./tmcast.py rx6
	joining ff1e::1 @ [7]mcast-rx
	rx: A.1
	rx: A.2
	rx: A.3

and similarly for also B, C or D being sender and all that with also IPv4.

Please see inside for details.

See also the following patch for more in-depth debugging of used approach.

So that we clearly see problems if packets are looped wrongly.

Or else cancellation does not work.
Amends 334a148f (tmcast.py: Handle SIGINT/SIGTERM).

linear:

	B--A--C

triangle:

	B-----A
	 \   /
	  \ /
	   C

So that smcrouted could be run separately on each.

See nexedi/nxdtest@b0cf277d for details
and for why signal handling is implemented like this.