libct: move cmsg helpers to new internal/cmsg package

These helpers all make more sense as a self-contained package and moving
them has the added benefit of removing an unneeded libpathrs dependency
(from libcontainer/utils's import of pathrs-lite) from several test
binaries.

Signed-off-by: Aleksa Sarai <aleksa@amutable.com>

tty.go

@@ -9,8 +9,8 @@ import (
 	"sync"
 
 	"github.com/containerd/console"
+	"github.com/opencontainers/runc/internal/cmsg"
 	"github.com/opencontainers/runc/libcontainer"
-	"github.com/opencontainers/runc/libcontainer/utils"
 )
 
 type tty struct {
@@ -100,7 +100,7 @@ func (t *tty) initHostConsole() error {
 }
 
 func (t *tty) recvtty(socket *os.File) (Err error) {
-	f, err := utils.RecvFile(socket)
+	f, err := cmsg.RecvFile(socket)
 	if err != nil {
 		return err
 	}