cksum: use more exact selection of digest algorithms

cksum: use more exact selection of digest algorithms

Use more constrained argument matching
to improve forward compatibility and robustness.

For example it's better that `cksum -a sha3` is _not_
equivalent to `cksum -a sha386`, so that a user
specifying `-a sha3` on an older cksum would not be surprised.

Also argmatch() is used when parsing tags from lines like:
SHA3 (filename) = abcedf....
so it's more robust that older cksum instances to fail
earlier in the parsing process, when parsing output from
possible future cksum implementations that might support SHA3.

* src/digest.c (algorithm_from_tag): Use argmatch_exact()
to ensure we don't match abbreviated algorithms.
(main): Likewise.
* tests/misc/cksum-a.sh: Add a test case.