Remove main package dependency on criurpc

Commit 7f64fb47 made the main package, and runc/libcontainer's CriuOpts
depend on criu/rpc. This is not good; among the other things, it makes
it complicated to make c/r optional.

Let's switch CriuOpts.ManageCgroupsMode to a string (yes, it's an APIt
breaking change) and move the cgroup mode string parsing to
libcontainer.

While at it, let's better document ManageCgroupsMode.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

checkpoint.go

@@ -8,7 +8,6 @@ import (
 	"path/filepath"
 	"strconv"
 
-	criu "github.com/checkpoint-restore/go-criu/v6/rpc"
 	"github.com/moby/sys/userns"
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/sirupsen/logrus"
@@ -132,6 +131,7 @@ func criuOptions(context *cli.Context) (*libcontainer.CriuOpts, error) {
 		StatusFd:                context.Int("status-fd"),
 		LsmProfile:              context.String("lsm-profile"),
 		LsmMountContext:         context.String("lsm-mount-context"),
+		ManageCgroupsMode:       context.String("manage-cgroups-mode"),
 	}
 
 	// CRIU options below may or may not be set.
@@ -152,21 +152,6 @@ func criuOptions(context *cli.Context) (*libcontainer.CriuOpts, error) {
 		}
 	}
 
-	switch context.String("manage-cgroups-mode") {
-	case "":
-		// do nothing
-	case "soft":
-		opts.ManageCgroupsMode = criu.CriuCgMode_SOFT
-	case "full":
-		opts.ManageCgroupsMode = criu.CriuCgMode_FULL
-	case "strict":
-		opts.ManageCgroupsMode = criu.CriuCgMode_STRICT
-	case "ignore":
-		opts.ManageCgroupsMode = criu.CriuCgMode_IGNORE
-	default:
-		return nil, errors.New("Invalid manage-cgroups-mode value")
-	}
-
 	// runc doesn't manage network devices and their configuration.
 	nsmask := unix.CLONE_NEWNET