Decode RDNSS addresses and keep their link scope

An RDNSS option has Type 25, an 8-bit Length measured in units of 8 octets including the header, a Reserved field, a 32-bit unsigned Lifetime in seconds, and one or more 128-bit IPv6 addresses. Reserved is zero in our chosen encoding; field widths are shown schematically (§5.1).

• An RDNSS option has Type 25, an 8-bit Length measured in units of 8 octets including the header, a Reserved field, a 32-bit unsigned Lifetime in seconds, and one or more 128-bit IPv6 addresses. Reserved is zero in our chosen encoding; field widths are shown schematically (§5.1).
• One address makes Length=3 and 24 bytes total: 8 header bytes plus 16 address bytes. Each additional address increases Length by 2; two addresses make Length=5 and 40 bytes. The receiver checks Length>=3 and (Length−1)%2==0; address count is (Length−1)/2 (§§5.1,5.3.1).
• All addresses in one option share its Lifetime. Use separate RDNSS options for different lifetimes. Invalid length cannot be repaired by silently treating bytes as another address; invalid options MUST be discarded. The addresses are checked for unicast suitability (§5.3.1). Our helper screens multicast/unspecified examples but is not a complete address or RA policy engine.
• Link-local RDNSS addresses MAY be advertised. Such entries SHOULD be registered with their receiving link’s zone index, and DNS queries to a link-local RDNSS MUST use that corresponding link (§5.1). fe80::53%linkA is a possible local text representation; %linkA is not included in the 128-bit on-wire address (RFC 4007 §11).
• Two links can have the same link-local address text while identifying different servers. The stored link distinguishes them. These codecs handle one already-framed option and selected hexadecimal IPv6 literals, not an entire RA, dotted-tail literal syntax, all special-address policy, routing or live DNS.