RFC: change cksum -b to allow full emulation of older utils

[Pádraig Brady]

It would be nice to be able to support users who want to replace all
the *sum utilities with a script that did:

  exec cksum -a <algo> --untagged "$@"

To that end, it would be good to support all the options of the older utils.
This is a bit awkward with --base64 added in the last few months, as that
also added the short -b option.  However it's only a few months released,
so it's probably not too onerous to change.

The attached makes the change, which I'll augment with NEWS and tests/ upon 
agreement.

cheers,
Pádraig

cksum-emulate.patch
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