Add ud_initialize helper function

Add ud_initialize helper function as it gives us more simple interface to
initialization sequence. Instead of calling:

  ud_init(&ud)

  ud_set_mode(&ud, m);
  ud_set_vendor(&ud, v);
  ud_set_input_buffer(&ud, buf, len);

  ... we can call now:

  ud_initialize(&ud, m, v, buf, len);