llist: remove direct struct accesses, use only functions

- Turned them all into functions to also do asserts etc.

- The llist related structs got all their fields renamed in order to make
  sure no existing code remains using direct access.

- Each list node struct now points back to the list it "lives in", so
  Curl_node_remove() no longer needs the list pointer.

- Rename the node struct and some of the access functions.

- Added lots of ASSERTs to verify API being used correctly

- Fix some cases of API misuse

Add docs/LLIST.md documenting the internal linked list API.

Closes #14485

lib/http.c

@@ -3118,7 +3118,7 @@ CURLcode Curl_http_header(struct Curl_easy *data,
 #ifdef DEBUGBUILD
       else
         infof(data, "Parsed STS header fine (%zu entries)",
-              data->hsts->list.size);
+              Curl_llist_count(&data->hsts->list));
 #endif
     }
 #endif