Revert recent checksum CLI changes

[RenjiSann]

This PR reverts several changes that transitioned manual CLI argument parsing, at the expense of error messages explicitness and GNU behavior soundness.

This is preparatory work for cksum/hashsum merging, which will greatly simplify the maintenance of checksum-related utils

[sylvestre]

some jobs are failing

[oech3]

No need to revert everything. --warn,etc... without --check only.
We can just backport GNU master's error handlings #10131

[RenjiSann]

No need to revert everything. --warn,etc... without --check only. We can just backport GNU master's error handlings #10131

I'm sorry it looks a bit severe, but I'm having a hard time making the difference between everything, partly because I wasn't following along in the last two weeks.

I admit that this may not be the most optimal solution, but I really want to be done with getting rid of hashsum and code duplication already, so I'd rather revert too many things now and then reconsider all these changes once we have a common base to work on for all tools.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[oech3]

OK. I made every my *sum PR to draft. Let delete hashsum anyway.