face: Making environment-based selection of the daemon (ndnd-tlv vs. nfd)

If ``NFD`` environment variable is set to any value, a ndn-cpp-dev
application will assume it is connected to nfd and will invoke
appropriate prefix registration protocol.

Change-Id: I33f3a3a29e17b5b5ddd9474427036402d9d657e3
diff --git a/src/face.hpp b/src/face.hpp
index 42f4fd1..9b1fbe4 100644
--- a/src/face.hpp
+++ b/src/face.hpp
@@ -57,27 +57,29 @@
   /**
    * @brief Create a new Face for communication with an NDN Forwarder using the default UnixTransport.
    */
-  Face(bool nfdMode = false);
+  Face();
 
   /**
    * @brief Create a new Face for communication with an NDN Forwarder using the default UnixTransport.
    * @param ioService A shared pointer to boost::io_service object that should control all IO operations
    */
-  Face(const ptr_lib::shared_ptr<boost::asio::io_service> &ioService, bool nfdMode = false);
+  explicit
+  Face(const ptr_lib::shared_ptr<boost::asio::io_service> &ioService);
   
   /**
    * Create a new Face for communication with an NDN hub at host:port using the default TcpTransport.
    * @param host The host of the NDN hub.
    * @param port The port or service name of the NDN hub. If omitted. use 6363.
    */
-  Face(const std::string &host, const std::string &port = "6363", bool nfdMode = false);
+  Face(const std::string &host, const std::string &port = "6363");
 
   /**
    * Create a new Face for communication with an NDN hub with the given Transport object and connectionInfo.
    * @param transport A shared_ptr to a Transport object used for communication.
    * @param transport A shared_ptr to a Transport::ConnectionInfo to be used to connect to the transport.
    */
-  Face(const shared_ptr<Transport>& transport, bool nfdMode = false);
+  explicit
+  Face(const shared_ptr<Transport>& transport);
 
   /**
    * @brief Alternative (special use case) version of the constructor, can be used to aggregate
@@ -92,8 +94,7 @@
    * </code>
    */
   Face(const shared_ptr<Transport>& transport,
-       const shared_ptr<boost::asio::io_service> &ioService,
-       bool nfdMode = false);
+       const shared_ptr<boost::asio::io_service> &ioService);
   
   /**
    * @brief Express Interest
@@ -194,7 +195,7 @@
 
 private:
   void
-  construct(const shared_ptr<Transport>& transport, const shared_ptr<boost::asio::io_service> &ioService, bool nfdMode);
+  construct(const shared_ptr<Transport>& transport, const shared_ptr<boost::asio::io_service>& ioService);
   
   struct ProcessEventsTimeout {};
   typedef std::list<shared_ptr<PendingInterest> > PendingInterestTable;