rem: return tail_lines (pretty_filename, fd, (long) n_units);
add: return tail_lines (filename, fd, n_units, read_pos);
context: tail (const char *pretty_filename, int fd, off_t n_units){  if (count_lines)    return tail_lines (pretty_filename, fd, (long) n_units);  else    return tail_bytes (pretty_filename, fd, n_units);}