lib: Mark signal waiting IO as "never wait alone"

This signal is being automatically moved to nested ioloops. It can't be the
only IO being waited, since there's no code that simply wants to wait for a
signal.

src/lib/lib-signals.c

@@ -301,6 +301,7 @@ lib_signals_enable_delayed_hander(void)
 	if (current_ioloop != NULL) {
 		io_sig = io_add(sig_pipe_fd[0], IO_READ,
 			signal_read, NULL);
+		io_set_never_wait_alone(io_sig, TRUE);
 	}
 }