Convert CLI to urfave v2

Followed the Migration Guide at https://cli.urfave.org/migrate-v1-to-v2/
The major changes not pointed out in the migration guide are:
- context.Args() no longer produces a []slice, so context.Args().Slice()
  in substitued
- All cli.Global***** are deprecated (the migration guide is somewhat
  unclear on this)

Signed-off-by: Derek Nola <derek.nola@suse.com>

Vendor in urfave cli/v2

Signed-off-by: Derek Nola <derek.nola@suse.com>

Fix NewStringSlice calls

Signed-off-by: Derek Nola <derek.nola@suse.com>

cmd/ctr/commands/containers/restore.go

@@ -26,19 +26,19 @@ import (
 	"github.com/containerd/containerd/v2/pkg/cio"
 	"github.com/containerd/errdefs"
 	"github.com/containerd/log"
-	"github.com/urfave/cli"
+	"github.com/urfave/cli/v2"
 )
 
-var restoreCommand = cli.Command{
+var restoreCommand = &cli.Command{
 	Name:      "restore",
 	Usage:     "Restore a container from checkpoint",
 	ArgsUsage: "CONTAINER REF",
 	Flags: []cli.Flag{
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "rw",
 			Usage: "Restore the rw layer from the checkpoint",
 		},
-		cli.BoolFlag{
+		&cli.BoolFlag{
 			Name:  "live",
 			Usage: "Restore the runtime and memory data from the checkpoint",
 		},
@@ -136,7 +136,7 @@ var restoreCommand = cli.Command{
 			return err
 		}
 		if code != 0 {
-			return cli.NewExitError("", int(code))
+			return cli.Exit("", int(code))
 		}
 		return nil
 	},