Add -C to sha256sum and other cksum tool (similar to tar)

[Przemysław Sztoch via GNU coreutils General Discussion]

Please add -C, --directory=DIR to sha256/512sum with similar action as 
in tar command:
Change to DIR before performing any operations.  This option is 
order-sensitive, i.e. it affects all options that follow.

Second sentence about order-sensitive is optional, but will be very 
useful sometimes.
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