bug#6600: [PATCH] sort: add --threads option to parallelize internal sort.

[Pádraig Brady]

I've finally applied the patch.
http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commit;h=9face836

I made a few comment tweaks and added
some dependencies for the heap module.

I also removed the xmemcoll0() calls
which are separate to this concurrent functionality.
I will add those back in Chen's name after updating to
the latest gnulib.

Thanks everyone for their work on this!

Pádraig.