Block allocator performance fix: instead of keeping the free list
ordered, keep it doubly-linked, and introduce a new flag BF_FREE so we
can tell when a block is free.  We can still coalesce blocks on the
free list because block descriptors are kept consecutively in memory,
so we can tell based on the BF_FREE flag whether to coalesce with the
next higher/lower blocks when freeing a block.

This (almost) make freeChain O(n) rather than O(n^2), and has been
reported to help a lot when dealing with very large heaps.