Fix some memory leaks in the slab allocator (funny, isn't it?).

alloc-slab.c (SLAB_DEBUG): Allow environment to override definition.
  (SLAB_RESERVE): Likewise.
  (MAX_SLAB_FREE): New preprocessor macro, used to cap free_slab_count.
  (slab_unalloc): Reindent SLAB_RESERVE logic.  Unconditionally link the
    free slab to the list of free slabs.  Add code to check MAX_SLAB_FREE.