checkpoint: add support for containers with terminals

CRIU was extended to report about orphaned master pty-s via RPC.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

restore.go

@@ -20,6 +20,11 @@ restored.`,
 	Description: `Restores the saved state of the container instance that was previously saved
 using the runc checkpoint command.`,
 	Flags: []cli.Flag{
+		cli.StringFlag{
+			Name:  "console-socket",
+			Value: "",
+			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the master end of the console's pseudoterminal",
+		},
 		cli.StringFlag{
 			Name:  "image-path",
 			Value: "",