Run a verb from the controller's own running image, and refuse what it cannot run as a handover (hq issue 289)

The witness moves a running build aside into a directory the controller's user
cannot enter, then deletes it; a verb exec'd from os.Executable() in that window
failed with permission denied. /proc/self/exe stays valid while the process lives.
A verb that still cannot start, or arrives while the controller stops, is refused
with link.ErrHandingOver and marked retry: handing-over.

cmd/mesh-controller/push.go

@@ -128,6 +128,10 @@ func serve(ctx context.Context) (err error) {
 			stopActing()
 		case <-ctx.Done():
 		}
+		// Stopping, whichever way: a verb arriving from here is refused as a handover, so its caller
+		// asks the controller after this one rather than have a command started and killed with this
+		// process (novox/hq issue 289).
+		handingOver.Store(true)
 	}()
 	defer func() {
 		select {