Implement socket_base_t::get_credential member function

The get_credential () member function returns
credential for the last peer we received message for.
The idea is that this function is used to implement user-level API.

src/msg.hpp

@@ -49,6 +49,7 @@ namespace zmq
         {
             more = 1,           //  Followed by more parts
             command = 2,        //  Command frame (see ZMTP spec)
+            credential = 32,
             identity = 64,
             shared = 128
         };
@@ -70,6 +71,7 @@ namespace zmq
         int64_t fd ();
         void set_fd (int64_t fd_);
         bool is_identity () const;
+        bool is_credential () const;
         bool is_delimiter () const;
         bool is_vsm ();
         bool is_cmsg ();