Set socket buffers before establishing TCP connection

The window scale option carried in SYN segment is computed from socket's
receive buffer size. So we need to set this buffer size before calling
connect or bind.

src/tcp_connecter.cpp

@@ -226,6 +226,12 @@ int zmq::tcp_connecter_t::open ()
     // Set the socket to non-blocking mode so that we get async connect().
     unblock_socket (s);
 
+    //  Set the socket buffer limits for the underlying socket.
+    if (options.sndbuf != 0)
+        set_tcp_send_buffer (s, options.sndbuf);
+    if (options.rcvbuf != 0)
+        set_tcp_receive_buffer (s, options.rcvbuf);
+
     //  Connect to the remote peer.
     int rc = ::connect (
         s, addr->resolved.tcp_addr->addr (),