fix residual space in memory block

 Here is the patch that create from residual space in memory block new free
chunks. We already discussed about it during 7.0. --- I owe this patch :-)

 My previous implementation create _one_ chunk only. Now I implement it
in do{}while, and if residual space is large, it create more free chunks.
It is better for memory utilize.

 For example in block is 112b (residual and in current code never used)

 first new chunk  64 + 8 (header)
 second new chunk  32 + 8 (header)

 Tom, is it right?

                        Karel