ln: accept non-utf8 source names in target-dir modes

[can1357]

When given target-directory forms like ln SOURCE... DIRECTORY, uutils ln rejects source paths with non-UTF-8 filename bytes, while GNU ln accepts them and creates the link in the destination directory.

Reproduction Steps

name=$(printf 'bad_\377'); mkdir dst; : > "$name"; ln "$name" dst
# Expected (GNU): exit 0, creates dst/bad_\377
# Actual (uutils): exit 1 with "cannot stat $'bad_\xFF'", dst remains empty

Impact

GNU compatibility is broken for byte-oriented filenames on Unix filesystems.

[xtqqczze]

Nice! I guess we can also remove unused ln-error-cannot-stat strings.