Add List head/node distinction as TODO

I already said it. There must be a convenient way - I currently only see the
possibility to create a different type for one of them - to distinguish at
compile time between list head and an ordinary node. It's necessary since
not even I can consistently use the list functions correctly (but only when
it's late, though :-)) Frankly, changing the type of list head or node
to get errors at compile time imposes more work than I want to do for it,
so... :-)