Create server cookies for clients that send client cookies.
Needs to be turned on in the config file with:
	answer-cookie: yes
A cookie-secret can be configured for anycast setups.
Also adds an access control list that will allow queries with
either a valid cookie or over a stateful transport.

…heir own
  file in util/rfc_1982.[ch].

  tests.

  Cookies.

…figparser.c, configparser.h).

  by allowing Cookies to bypass the ratelimit, but still allowing
  ratelimit to valid DNS Cookie clients via the new
  ip-ratelimit-cookie option.

  thread and total: num.queries_cookie_valid, num.queries_cookie_client,
  num.queries.cookie_invalid.

  8 bytes after a client option of length 8, and then ignore them to
  recreate a 24 byte response. The fixup does not read out of bounds,
  and puts zeroes in the buffer at that point, that then are ignored.