unexpand: rename uflag to utf8

[cakebaker]

This PR renames all uses of uflag to utf8. The name uflag is misleading because the variable is the inverse of the flag. I used the name utf8 to match the name we use in expand for the same concept.

[Alonely0]

makes sense. grepping the codebase I also got 6 occurrences in id. Wanna go for it as well?

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/basenc/bounded-memory is now passing!

[cakebaker]

I would do it in a different PR, as there the name is not used in a misleading way.