uniq: fix -w to count bytes in C locale

[aguimaraes]

Summary

uniq -w N should count bytes in C/POSIX locale and characters in UTF-8 locale. Currently it always counts UTF-8 characters regardless of locale.

Changes

• Added is_c_locale() helper that checks LC_ALL, LC_CTYPE, LANG in order
• Modified key_end_index() to use byte counting when in C locale
• Added test for C locale byte counting behavior
• Fixed test_stdin_w1_multibyte to explicitly set UTF-8 locale (it was implicitly relying on character counting)

Considerations

I chose to inline the locale check (~9 lines) rather than adding the i18n feature dependency. The check is simple enough that duplicating it seemed better than pulling in ICU dependencies just for this.

If you'd prefer I use uucore::i18n instead, let me know and I'll update.

Fixes #10184

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)