MINOR: signal: really ignore signals configured with no handler

Until now, signals configured with no handler were still enabled and
ignored upon signal reception. Until now it was not an issue but with
SSL causing many EPIPE all the time, it becomes obvious that signal
processing comes with a cost. So set the handler to SIG_IGN when the
function is NULL.

src/signal.c

@@ -144,7 +144,7 @@ struct sig_handler *signal_register_fct(int sig, void (*fct)(struct sig_handler
 		return NULL;
 
 	if (sig)
-		signal(sig, signal_handler);
+		signal(sig, fct ? signal_handler : SIG_IGN);
 
 	if (!fct)
 		return NULL;