Use NULL instead of END_X_QUEUE closures?

[Ömer Sinan Ağacan]

Currently we sometimes use special closures to mark end of lists of different
objects. Some examples:

- END_TSO_QUEUE
- END_STM_WATCH_QUEUE
- END_STM_CHUNK_LIST

But we also use NULL for the same thing, e.g. in weak pointer lists
(old_weak_ptr_list, weak_ptr_list).

I'm wondering why we need special marker objects (which are actual closures
with info tables) instead of using NULL consistently. Current approach causes a
minor problem when working on the RTS because every time I traverse a list I
need to remember how the list is terminated (e.g. NULL when traversing weak
pointer lists, END_TSO_QUEUE when traversing TSO lists).

Ömer