I would like a more constrained argument matching to improve forward compatibility and robustness.
For example I would like `cksum -a sha3` to _not_ be 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.
