Use ParseSignal and AtomicWriteFile functions from containerd

Containerd has its own ParseSignal and AtomicWriteFile implementation.
So there's no need to use these function from github.com/docker/docker.

Signed-off-by: Shengjing Zhu <i@zhsj.me>

pkg/server/container_stop.go

@@ -22,7 +22,6 @@ import (
 	"github.com/containerd/containerd"
 	eventtypes "github.com/containerd/containerd/api/events"
 	"github.com/containerd/containerd/errdefs"
-	"github.com/docker/docker/pkg/signal"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/net/context"
@@ -134,7 +133,7 @@ func (c *criService) stopContainer(ctx context.Context, container containerstore
 				}
 			}
 		}
-		sig, err := signal.ParseSignal(stopSignal)
+		sig, err := containerd.ParseSignal(stopSignal)
 		if err != nil {
 			return errors.Wrapf(err, "failed to parse stop signal %q", stopSignal)
 		}