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/pull.hpp

@@ -46,6 +46,7 @@ namespace zmq
         void xattach_pipe (zmq::pipe_t *pipe_, bool subscribe_to_all_);
         int xrecv (zmq::msg_t *msg_);
         bool xhas_in ();
+        blob_t get_credential () const;
         void xread_activated (zmq::pipe_t *pipe_);
         void xpipe_terminated (zmq::pipe_t *pipe_);