Fix #1285 - __builtin___snprintf_chk warning. Tighten test flags and refactor the code to squash all warnings

docs/README.md

@@ -1015,19 +1015,18 @@ Subscribe to topic `topic`.
 ### mg\_mqtt\_next\_sub()
 
 ```c
-int mg_mqtt_next_sub(struct mg_mqtt_message *msg, struct mg_str *topic,
-                     uint8_t *qos, int pos);
+size_t mg_mqtt_next_sub(struct mg_mqtt_message *msg, struct mg_str *topic, uint8_t *qos, size_t pos);
 ```
 
 Traverse list of subscribed topics. 
 Used to implement MQTT server when `MQTT_CMD_SUBSCRIBE` is received.
-Return next position. Initial position `pos` should be 4. Example:
+Return next position, or 0 when done. Initial position `pos` should be 4. Example:
 
 ```c
 if (ev == MG_EV_MQTT_CMD) {
   struct mg_mqtt_message *mm = (struct mg_mqtt_message *) ev_data;
   if (mm->cmd == MQTT_CMD_SUBSCRIBE) {
-    int pos = 4;
+    size_t pos = 4;
     uint8_t qos;
     struct mg_str topic;
     while ((pos = mg_mqtt_next_sub(mm, &topic, &qos, pos)) > 0) {
@@ -1040,8 +1039,7 @@ if (ev == MG_EV_MQTT_CMD) {
 ### mg\_mqtt\_next\_unsub()
 
 ```c
-int mg_mqtt_next_unsub(struct mg_mqtt_message *msg, struct mg_str *topic,
-                       int pos);
+size_t mg_mqtt_next_unsub(struct mg_mqtt_message *msg, struct mg_str *topic, size_t pos);
 ```
 
 Same as `mg_mqtt_next_sub()`, but for unsubscribed topics. The difference