cksum: prefer -a sha2 -l ###, to -a sha###

cksum: prefer -a sha2 -l ###, to -a sha###

To make the interface more concise and consistent,
while being backwards compatible.

* src/digest.c (main): Continue to support -a "sha###" but
also support -a "sha2" and treat it like "sha3", except in...
(output_file): ... maintain the legacy tags for better compatability.
* doc/coreutils.texi (cksum invocation): Document the -a sha2 option.
* tests/cksum/cksum-base64.pl: Adjust as per modified --help.
* tests/cksum/cksum-c.sh: Add new supported SHA2-### tagged variant.
* NEWS: Mention the new feature.