Fix signalpipe file descriptor leak on RESTART

Signals_Init() must only be called once.
This does not affect any ngircd release version.

Earlier version of this patch moved the io and sighandler
initialization before the while() loop, but as Alexander
Barton noticed that broke all systems without builtin select
support in io.c...

src/ngircd/sighandlers.c

@@ -266,6 +266,8 @@ Signals_Init(void)
 #ifdef HAVE_SIGACTION
 	struct sigaction saction;
 #endif
+	if (signalpipe[0] > 0 || signalpipe[1] > 0)
+		return true;
 
 	if (pipe(signalpipe))
 		return false;