Dont wait for pending subscription commands on shutdown

When (X)SUB socket is being shut down there may be pending
outbound subscription commands in the pipes. In such case
we want to close the socket immediately instead of waiting
for the commands to be sent.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/xsub.cpp

@@ -30,6 +30,11 @@ zmq::xsub_t::xsub_t (class ctx_t *parent_, uint32_t tid_) :
     more (false)
 {
     options.type = ZMQ_XSUB;
+
+    //  When socket is being closed down we don't want to wait till pending
+    //  subscription commands are sent to the wire.
+    options.linger = 0;
+
     int rc = message.init ();
     errno_assert (rc == 0);
 }