Stop a completed run from reporting that it was interrupted

[tamnd]

Found while running M14 (#18). Every pass ends by printing this, whether or not anybody signalled it:

campaign: 400 done, 0 failed, 0 skipped of 400 queued, 940 statements, 642 entailed, 96 in review, 2225619 tokens, 1h17m50s
draining, finishing the provisions already in flight, signal again to abort

Nobody signalled that run. It read all 400 provisions and then announced that it was being cut short.

The cause is the shape the signal handling was written in. signal.NotifyContext with a deferred stop and a goroutine waiting on ctx.Done means the normal return cancels the context too, the goroutine cannot tell the two apart, and the line goes out on the way past. The effect is that a log cannot distinguish a completed pass from an abandoned one, which is the only thing that line was ever for, and on passes that take hours the log is usually all anybody has.

The two ways out are separate channels now and the goroutine is told which one happened. The notice also goes to a writer the caller names rather than to os.Stderr directly: a goroutine reading a package level variable is a thing a test cannot get underneath, and the race detector says so rather than letting the test look like it passes.

Both the breadth front end and the older norm run had it, from the same copied block, so both are fixed and they share the helper now.

Two tests. The one that matters runs everywhere, because it is the case an ordinary passing run exercises. The one that sends a real SIGTERM is built out on Windows rather than skipped, since there is no signal there to send.

gofmt clean, go vet clean, golangci-lint 0 issues, go test -race ./... green, and GOOS=windows go build and go vet both clean.