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.cpp

@@ -267,6 +267,11 @@ bool zmq::msg_t::is_identity () const
     return (u.base.flags & identity) == identity;
 }
 
+bool zmq::msg_t::is_credential () const
+{
+    return (u.base.flags & credential) == credential;
+}
+
 bool zmq::msg_t::is_delimiter () const
 {
     return u.base.type == type_delimiter;