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/router.cpp

@@ -371,6 +371,11 @@ bool zmq::router_t::xhas_out ()
     return true;
 }
 
+zmq::blob_t zmq::router_t::get_credential () const
+{
+    return fq.get_credential ();
+}
+
 bool zmq::router_t::identify_peer (pipe_t *pipe_)
 {
     msg_t msg;