fix connection establishment for AF_UNIX sockets

src/ipc_connecter.cpp

@@ -128,7 +128,7 @@ void zmq::ipc_connecter_t::start_connecting ()
     }
 
     //  Connection establishment may be delayed. Poll for its completion.
-    else if (rc == -1 && errno == EAGAIN) {
+    else if (rc == -1 && errno == EINPROGRESS) {
         handle = add_fd (s);
         handle_valid = true;
         set_pollout (handle);