Fix the allocator.
Fix the allocator.

	It had two bugs that compensated each other. First,
	an off-by-one in the code that prevented freeing any
	slabs. Second, the singly linked list of slabs should
	have been doubly linked.

	The first prevented the second from mattering.

	Both should be fixed by this.

oridb/mc,oridb/mc,oridb/mc,oridb/mc