cangen: Follow Bash exit status when signaled

Bash and many other shells use 128 + signum as the exit status when a
program get signal and exit.  Follow the common behavior so that we
know how the programs are killed.

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>

cangen.c

@@ -91,6 +91,7 @@
 extern int optind, opterr, optopt;
 
 static volatile int running = 1;
+static volatile sig_atomic_t signal_num;
 static unsigned long long enobufs_count;
 static bool ignore_enobufs;
 static bool use_so_txtime;
@@ -220,6 +221,7 @@ static void print_usage(char *prg)
 static void sigterm(int signo)
 {
 	running = 0;
+	signal_num = signo;
 }
 
 static int setsockopt_txtime(int fd)
@@ -887,5 +889,8 @@ int main(int argc, char **argv)
 
 	close(s);
 
+	if (signal_num)
+		return 128 + signal_num;
+
 	return 0;
 }