uniq: pass remaining GNU tests

[zhitkoff]

This PR makes the following changes in order to pass GNU tests/uniq/uniq.pl:

1. Rework of how obsolete options are handled - following the same approach as was done for split. Unfortunately it is not possible to implement it with just Clap for all required test cases
2. In support of (1) above - introduces a module posix with 3 constants and a single function to return POSIX version based on _POSIX2_VERSION environment variable. This module would be needed to add similar functionality to sort, tail and touch later on (most likely to finalize their GNU tests compliance as well)
3. Rework of how input is processed - switching from requiring UTF8 (and failing if input has invalid UTF8 characters) to processing raw u8 bytes sequence. This is required to pass GNU schar test case and in general aligns the behavior with GNU version
4. Override some Clap errors to match the exact wording of error messages hardcoded into GNU tests
5. More tests to cover the changes

Should also fix #3509

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/uniq/uniq is no longer failing!

[zhitkoff]

@sylvestre checking in to see if you have any other comments / suggestions on this PR

[sylvestre]

what about using an enum here?

[zhitkoff]

yes, initially i tried to implement it as an enum, but discarded in favor of these constants - unfortunately it is inconsistently checked in GNU, so was easier to implement this way for now. Might need to give it another try later on as these will be needed to fully cover GNU tests for at least 2 more utilities

[sylvestre]

ok, thanks

[sylvestre]

Impressive work!