title: how to specify zmq swap file path
body: <p>I am using the ZeroMQ C++ libary. The <code>ZMQ_SWAP</code> option puts outstanding messages to storage on disk. How can I specify a swap file? The <code>zmq_setsokopt</code> function only sets swap size like this:</p>
  
  <pre><code>uint64_t swap = 25000000;
  zmq_setsockopt (publisher, ZMQ_SWAP, &amp;swap, sizeof (swap));
  </code></pre>