Add separate userdata for each http endpoint hdlr

PUBLISHED_FROM=29a204c2c7f35724b6f1e8435edb94869da2362c

docs/c-api/http_server.h/mg_register_http_endpoint.md

@@ -4,7 +4,8 @@ decl_name: "mg_register_http_endpoint"
 symbol_kind: "func"
 signature: |
   void mg_register_http_endpoint(struct mg_connection *nc, const char *uri_path,
-                                 mg_event_handler_t handler);
+                                 MG_CB(mg_event_handler_t handler,
+                                       void *user_data);
 ---
 
 Registers a callback for a specified http endpoint

examples/big_upload/big_upload.c

@@ -98,7 +98,7 @@ int main(void) {
   mg_mgr_init(&mgr, NULL);
   nc = mg_bind(&mgr, s_http_port, ev_handler);
 
-  mg_register_http_endpoint(nc, "/upload", handle_upload);
+  mg_register_http_endpoint(nc, "/upload", handle_upload MG_UD_ARG(NULL));
   // Set up HTTP server parameters
   mg_set_protocol_http_websocket(nc);
 

mongoose.h

@@ -4792,7 +4792,8 @@ void mg_file_upload_handler(struct mg_connection *nc, int ev, void *ev_data,
  * ```
  */
 void mg_register_http_endpoint(struct mg_connection *nc, const char *uri_path,
-                               mg_event_handler_t handler);
+                               MG_CB(mg_event_handler_t handler,
+                                     void *user_data));
 
 /*
  * Authenticates a HTTP request against an opened password file.