Problem: signature of zmq_poller_fd does is incompatible with regular error handling

Solution: change return type to int (again) and return fd via an output parameter

src/socket_poller.hpp

@@ -76,7 +76,7 @@ class socket_poller_t
     int modify_fd (fd_t fd_, short events_);
     int remove_fd (fd_t fd_);
     // Returns the signaler's fd if there is one, otherwise errors.
-    fd_t signaler_fd ();
+    int signaler_fd (fd_t *fd_);
 
     int wait (event_t *event_, int n_events_, long timeout_);