sha1sum collisions

[Zooko Wilcoz]

Folks:

As you've no doubt heard, there are now SHA1 collisions in the wild:
http://shattered.io/

(See also, if you're interested, https://z.cash/blog/hash-functions.html .)

The invocation docs for sha1sum should be updated to reflect this, and
also — as with the md5sum invocation docs — b2sum should be suggested
as an alternative.

Also, these web pages should be updated to newer a coreutils release
so that these recommendations are visible there:

https://www.gnu.org/software/coreutils/manual/html_node/md5sum-invocation.html

Regards,

Zooko