face: introduce NullLinkService and use it in makeNullFace()

Change-Id: Ia983c62fecdf94aa4f8cbb034735a229bd58d742
diff --git a/daemon/face/null-face.cpp b/daemon/face/null-face.cpp
index a793e3e..2ce5c98 100644
--- a/daemon/face/null-face.cpp
+++ b/daemon/face/null-face.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
- * Copyright (c) 2014-2015,  Regents of the University of California,
+/*
+ * Copyright (c) 2014-2019,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -24,19 +24,18 @@
  */
 
 #include "null-face.hpp"
-#include "generic-link-service.hpp"
+#include "null-link-service.hpp"
 #include "internal-transport.hpp"
 
 namespace nfd {
 namespace face {
 
-// FIB could restrict creating a nexthop record toward non-local face in /localhost namespace.
-// NullFace has scope=local to enable creating a "blackhole" FIB entry under /localhost.
-
 shared_ptr<Face>
 makeNullFace(const FaceUri& uri)
 {
-  return make_shared<Face>(make_unique<GenericLinkService>(),
+  // FIB could restrict creating a nexthop record toward a non-local face in /localhost namespace.
+  // Therefore, NullFace has scope=local to enable creating a "blackhole" FIB entry under /localhost.
+  return make_shared<Face>(make_unique<NullLinkService>(),
                            make_unique<InternalForwarderTransport>(uri, uri, ndn::nfd::FACE_SCOPE_LOCAL));
 }