ipv6: remove obsolete three-server DNS limit

The legacy limit matches RFC 6106's recommendation that hosts treat three learned RDNSS addresses as sufficient. That was host-side repository guidance, not a limit in the RADIUS, DHCPv6, or RDNSS encodings.

RFC 8106 obsoletes RFC 6106 and explicitly removes the recommendation to limit learned RDNSS addresses to three, leaving the number to local policy. It instead recommends the ability to store at least three addresses when DNS information comes from multiple sources.

Store both configured and per-session servers in reusable contiguous arrays. Start with a four-address, 64-byte allocation, grow geometrically, retain capacity across reload or reauthorization, and free session storage only at teardown. Bound advertised lists by actual DHCPv6 and router-advertisement packet space.

Warn when servers have to be dropped for lack of room, as the fixed limit used to, skip the DNS search list rather than write it past the end of a full advertisement, and drop the stale three-server wording from the [ipv6-dns] documentation.

Link: https://github.com/accel-ppp/accel-ppp/commit/4f562467dbdf819395e138617c2a057e02595b9e#r197212083
Link: https://www.rfc-editor.org/rfc/rfc6106.html#section-5.3.1
Link: https://www.rfc-editor.org/rfc/rfc8106.html#section-5.3.1
Link: https://www.rfc-editor.org/rfc/rfc8106.html#appendix-A

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