argmatch: add variants that only match full argument

[Pádraig Brady]

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.