add: help message

src/main.c

@@ -93,7 +93,11 @@ int main(int ac, char **av)
     struct statistics stats;
     struct setting settings;
 
-    if (get_setting(av + 1, &settings))
+    int ret = get_setting(av + 1, &settings);
+
+    if (ret == -1)
+        goto finish;
+    if (ret)
         goto error0;
 
     size_t packet_size = sizeof(struct icmphdr) + settings.payload_size;
@@ -168,6 +172,7 @@ int main(int ac, char **av)
 
     print_statistics(&stats, &settings);
 
+finish:
     return 0;
 
 error3: