Ignore SIGURG signals in signal forwarder

Starting with go1.14, the go runtime hijacks SIGURG but with no way to
not send to other signal handlers.

In practice, we get this signal frequently.
I found this while testing out go1.15 with ctr and multiple execs with
only `echo hello`. When the process exits quickly, if the previous
commit is not applied, you end up with an error message that it couldn't
forward SIGURG to the container (due to the process being gone).

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cmd/ctr/commands/signals.go

@@ -37,6 +37,10 @@ func ForwardAllSignals(ctx gocontext.Context, task killer) chan os.Signal {
 	signal.Notify(sigc)
 	go func() {
 		for s := range sigc {
+			if canIgnoreSignal(s) {
+				logrus.Debugf("Ignoring signal %s", s)
+				continue
+			}
 			logrus.Debug("forwarding signal ", s)
 			if err := task.Kill(ctx, s.(syscall.Signal)); err != nil {
 				if errdefs.IsNotFound(err) {