llist: remove direct struct accesses, use only functions

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