Bump Libcontainer to latest head

vendor/github.com/opencontainers/runc/libcontainer/system/linux.go

@@ -100,17 +100,12 @@ func Setctty() error {
 	return nil
 }
 
-/*
- * Detect whether we are currently running in a user namespace.
- * Copied from github.com/lxc/lxd/shared/util.go
- */
+// RunningInUserNS detects whether we are currently running in a user namespace.
+// Copied from github.com/lxc/lxd/shared/util.go
 func RunningInUserNS() bool {
 	file, err := os.Open("/proc/self/uid_map")
 	if err != nil {
-		/*
-		 * This kernel-provided file only exists if user namespaces are
-		 * supported
-		 */
+		// This kernel-provided file only exists if user namespaces are supported
 		return false
 	}
 	defer file.Close()