ct_encode_char
used = ct_encode_char(dst, (size_t)5, *s);
libedit_private ssize_t ct_encode_char(char *, size_t, wchar_t);
ssize_t n = ct_encode_char(b, (size_t)(eb - b), *p2++);
n += ct_encode_char(b + n, (size_t)(w - n), buf[i]);
n += ct_encode_char(b + n, (size_t)(w - n), end[1]);
i = ct_encode_char(buf, (size_t)MB_LEN_MAX, c);