- Rename to the (more correct) NUM_FREE_LISTS

- NUM_FREE_LISTS should be derived from the block and mblock sizes, not
  defined manually.  It was actually too large by one, which caused a
  little bit of (benign) extra work in the form of a redundant loop
  iteration in some cases.

- Add some ASSERTs for input preconditions to log_2() and log_2_ceil()

- Fix some comments

- Fix usage in allocLargeChunk, to account for the fact that
  log_2_ceil() can return NUM_FREE_LISTS.