cksum: allow -a {blake2b,sha2,sha3} --check to work on base64

cksum: allow -a {blake2b,sha2,sha3} --check to work on base64

* NEWS: Mention the bug.
* src/digest.c (split_3): Check that the base64 digest matches the
length supported by the algorithm.
(digest_check): Check that the read digest matches the base64 length of
the algorithm's digest. The previous condition would not work for
'cksum -a blake2b -l 8 ...'.
* tests/cksum/cksum-base64-untagged.sh: New file.
* tests/local.mk (all_tests): Add the new test.