ln: rejects non-UTF-8 source filenames in target-directory mode

In target-directory forms (ln SOURCE... DIRECTORY), ln rejects source paths with non-UTF-8 filename bytes, while GNU accepts them. Breaks GNU compatibility for byte-oriented filenames on Unix filesystems.

PoC:

name=$(printf 'bad_\377'); mkdir dst; : > "$name"; ln "$name" dst
# GNU: exit 0, creates dst/bad_\377 ; uutils: exit 1, dst empty

Zellic private finding (zellic-ext/coreutils-private PR #84). Reported in the Zellic uutils coreutils Program Security Assessment (for Canonical, Jan 2026), audited commit 3a07ffc5a9bd4c283e75afa548ba1f1957bad242.