Enable checkpoint/restore of containers with tty

CRIU supports checkpoint and restore of tty devices since version 2.12
which was released on 8th of March 2017. Support for this functionality
was implemented with opencontainers/runc@1c43d09 (checkpoint: add
support for containers with terminals) and containerd/containerd@60daa41
(Allow to checkpoint and restore a container with console).

Therefore, we can enable the support in moby/docker.

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>

daemon/checkpoint.go

@@ -63,10 +63,6 @@ func (daemon *Daemon) CheckpointCreate(name string, config types.CheckpointCreat
 		return fmt.Errorf("Container %s not running", name)
 	}
 
-	if container.Config.Tty {
-		return fmt.Errorf("checkpoint not support on containers with tty")
-	}
-
 	if !validCheckpointNamePattern.MatchString(config.CheckpointID) {
 		return fmt.Errorf("Invalid checkpoint ID (%s), only %s are allowed", config.CheckpointID, validCheckpointNameChars)
 	}