bug#6643: [PATCH] sort: fix a bug with sort -u and xmemcoll0, and tune keycompare

[Chen Guo]

Hi Professor Eggert,
> Chen, can you please verify that "sort -u" does not access the same
> line from multiple threads, even saved lines?  If it does, then even this 
> patch
> is not enough: we would need to alter write_bytes so that it does not
> modify its argument at all.
Off the top of my head, no line is ever accessed by multiple threads
at the same time.
I'll go through the code tonight when I get home to doubly make sure.