Problem: IO thread name starts counting at 1

Solution: start at 0

src/io_thread.cpp

@@ -58,7 +58,7 @@ void zmq::io_thread_t::start ()
 {
     char name[16];
     snprintf (name, sizeof (name), "IO/%u",
-              get_tid () - zmq::ctx_t::reaper_tid);
+              get_tid () - zmq::ctx_t::reaper_tid - 1);
     //  Start the underlying I/O thread.
     _poller->start (name);
 }