Bug#518696: coreutils parallel enhancements

[Pádraig Brady <P@draigBrady.com>]

Just a note to say recent coreutils include an `nproc`
command which you can use like:

xargs -P$(nroc)
xargs -P$(nroc --ignore=1)

Also I've updated md5sum, sha1sum, ... to output
their lines atomically, so they're not interspersed.

cheers,
Pádraig.