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.