Introduce Denial-of-Existence (DoE) for Nack response.

Note this commit changes how names are stored in the database, compliant
to the canonical order.

Change-Id: I9857aaefc1f7da08ff53eff43c8f8c8bd5443953
Refs: #4152
diff --git a/src/ndns-label.hpp b/src/ndns-label.hpp
index 8ce7d34..076d06c 100644
--- a/src/ndns-label.hpp
+++ b/src/ndns-label.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2017, Regents of the University of California.
+ * Copyright (c) 2014-2018, Regents of the University of California.
  *
  * This file is part of NDNS (Named Data Networking Domain Name Service).
  * See AUTHORS.md for complete list of NDNS authors and contributors.
@@ -74,6 +74,10 @@
  */
 const name::Component TXT_RR_TYPE("TXT");
 
+/**
+ * @brief Denial of Existance record type
+ */
+const name::Component DOE_RR_TYPE("DOE");
 
 //////////////////////////////////////////
 
@@ -115,7 +119,6 @@
           const Name& zone,
           MatchResult& result);
 
-
 } // namespace label
 } // namespace ndns
 } // namespace ndn