Convert an &OsStr produced by POSIX-style APIs into a Cow<[u8]> which
is either plain UTF-8 or an ARF encoding. Returns an error if the input
string contains NUL bytes.
Convert an &OsStr produced by POSIX-style APIs into a Cow<str> which
is either plain UTF-8 or an ARF encoding. Returns an error if the input
string contains NUL bytes.