Fixed setting of max packet size to response.

- If both max size and OPT RR are given, the payload in the OPT RR
  cannot be larger than the max size - added check.
- Fixed API of response_add_opt().
- Disabled reallocation in response_add_opt() - larger size in OPT
  RR should not override the max size already set (max size is
  more important).

refs #769