timeout(1): fix the acceptable range values for parse_signal()

Before both 0 and sys_nsig would be successfully returned by parse_signal()
although being invalid signal numbers.

PR:		Alexandre Perrin <alex@kaworu.ch>
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D3990

usr.bin/timeout/timeout.c

@@ -105,7 +105,7 @@ parse_signal(const char *str)
 	int sig, i;
 	const char *errstr;
 
-	sig = strtonum(str, 0, sys_nsig, &errstr);
+	sig = strtonum(str, 1, sys_nsig - 1, &errstr);
 
 	if (errstr == NULL)
 		return (sig);