Be more conservative about when to generate ZMQ_EVENT_CLOSE_FAILED

This also fixes a bug in tcp_connecter and tcp_listener, which
generated the event not when they failed to close the socket but
when the succeed to close it.

src/ipc_listener.cpp

@@ -169,10 +169,7 @@ int zmq::ipc_listener_t::close ()
 {
     zmq_assert (s != retired_fd);
     int rc = ::close (s);
-    if (rc != 0) {
-        socket->monitor_event (ZMQ_EVENT_CLOSE_FAILED, endpoint.c_str(), zmq_errno());
-        return -1;
-    }
+    errno_assert (rc == 0);
 
     //  If there's an underlying UNIX domain socket, get rid of the file it
     //  is associated with.