date: Rework argument processing

[RenjiSann]

This pull request addresses several points on the argument processing of date:

• Enfore the mutual exclusivity of --date, --file, --reference, and --resolution (not yet implemented).
• Successive --date, --file and --reference arguments override their previous introduction.

This is still work in progress, as I also would like to fix the distinction between FORMAT positional arguments (starting with +) and date positional arguments.

[github-actions[bot]]

GNU testsuite comparison:

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

[BenWiederhake]

Doesn't this duplicate some of the work in https://github.com/uutils/coreutils/pull/6142 ?

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/rm/rm2 is no longer failing!

[sylvestre]

@RenjiSann it is still marked as draft, are you still working on it? thanks

[RenjiSann]

I am not working on this anymore. I don't really remember the context of it either. First, I'll try to finish the changes I want to bring to cksum before looking into another uutil.