[PATCH v5] libio: Fix ungetwc operating on byte stream [BZ #33998]

[Rocket Ma]

> One last comment I want to make:
> - For DCO'd contributions we should not include "(C) 2026" since thsi is not
>    a copyright statement, and as such I'm going to clean up the 3 of these that
>    have made it into the tree in the next commit.

Oh, got it. Actually when I looking at glibc wiki, I didn't see this
convention...

Rocket