cksum: permit repeated flags, handle overrides correctly, implement base64 output

[BenWiederhake]

This PR changes and tests three things:

• Accept --tag and --untagged in any order, use the last occurrence.
• Accept repetitions of everything, flags and arguments like --algorithm.
• Accept and correctly handle --base64 flag.

All three of these are GNU behavior bugs, i.e. GNU does it differently, and uutils wants to handle it like GNU does.

This is work towards #5998.

Note that cksum is still woefully incomplete; most prominently, the -c option is missing.

[BenWiederhake]

Changes since last push:

• Removed commented-out test code. Thanks @sylvestre for spotting it!
• Double-checked all the other commits
• Rebased on current main

[BenWiederhake]

Only now I see that a similar, but older PR from @tertsdiepraam exists: #5122
I'm not sure what's a good way forward.

[tertsdiepraam]

Yours can take priority. I haven't looked at mine in a while and I can always rebase it.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/chown/preserve-root is no longer failing!

[BenWiederhake]

Changes since last push: Just rebased, to demonstrate that it's still working.

Is there anything missing?

[sylvestre]

no, it is great, just busy. I will accept it once the CI is green

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)