node: Implement prefix 'unreg' when interest filter is unset

Filter unsetting is useful in some applications that want to terminate
after data has been successfully pushed out to the forwarding daemon.

Change-Id: Iabada0db7d1bd653899941a294abd4c8cd20a80f
diff --git a/include/ndn-cpp/forwarding-entry.hpp b/include/ndn-cpp/forwarding-entry.hpp
index 662409a..e9a3f47 100644
--- a/include/ndn-cpp/forwarding-entry.hpp
+++ b/include/ndn-cpp/forwarding-entry.hpp
@@ -22,9 +22,9 @@
 public:    
   ForwardingEntry(const std::string& action,
                   const Name& prefix,
-                  int faceId,
-                  const ForwardingFlags& forwardingFlags,
-                  int freshnessPeriod) 
+                  int faceId = -1,
+                  const ForwardingFlags& forwardingFlags = ForwardingFlags(),
+                  int freshnessPeriod = -1) 
     : action_(action)
     , prefix_(prefix)
     , faceId_(faceId)