security: Identity/Key comparison operators

This commit adds ==, !=, and stream insertion operators to
Identity and Key types, so that `ndnsec list` can display
default identity and key properly.

Also, a missing assignment is added to `ndnsec list` routine.

refs #4085

Change-Id: I025af1d6281acb9e9261461520bbd74af53b811e
diff --git a/tools/ndnsec/list.cpp b/tools/ndnsec/list.cpp
index 36026d7..955fb6f 100644
--- a/tools/ndnsec/list.cpp
+++ b/tools/ndnsec/list.cpp
@@ -1,5 +1,5 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
  * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -157,7 +157,7 @@
   // TODO add API to check for default identity (may be from the identity itself)
   security::Identity defaultIdentity;
   try {
-    keyChain.getPib().getDefaultIdentity();
+    defaultIdentity = keyChain.getPib().getDefaultIdentity();
   }
   catch (const security::Pib::Error&) {
     // no default identity