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/transport/unix-transport.hpp b/src/transport/unix-transport.hpp
index 26621d7..dc85eac 100644
--- a/src/transport/unix-transport.hpp
+++ b/src/transport/unix-transport.hpp
@@ -12,24 +12,26 @@
 #include "transport.hpp"
 
 namespace ndn {
-  
+
 class UnixTransport : public Transport
 {
 public:
-  UnixTransport(const std::string &unixSocket = "/tmp/.ndnd.sock");
+  UnixTransport();
+
+  UnixTransport(const std::string& unixSocket);
   ~UnixTransport();
 
   // from Transport
-  virtual void 
-  connect(boost::asio::io_service &ioService,
-          const ReceiveCallback &receiveCallback);
-  
-  virtual void 
+  virtual void
+  connect(boost::asio::io_service& ioService,
+          const ReceiveCallback& receiveCallback);
+
+  virtual void
   close();
 
-  virtual void 
-  send(const Block &wire);
-  
+  virtual void
+  send(const Block& wire);
+
 private:
   std::string unixSocket_;