net: ipv6: Neigbor reachablity detection is not compliant with RFC 4861

[rlubos]

Describe the bug

Zepyr's implementation of IPv6 neighbor reachability detection is not fully compliant with RFC 4861 (Neighbor Discovery for IP version 6 (IPv6)
), the following issues has been detected:

• Receiving Router Advertisement with Source Link-Layer Address option should create a new STALE entry in the neighbor table, currently Zephyr creates INCOMPLETE and sends multicast NS packet (https://datatracker.ietf.org/doc/html/rfc4861#section-6.3.4)
• NS packets sent in PROBE state (reachability confirmation after neighbor goes STALE) should be sent as unicast packets, Zephyr sends multicast instead (https://datatracker.ietf.org/doc/html/rfc4861#section-7.3.3)
• received NA packets with no Target Link-Layer Address option should still be able to bring the neighbor state back to REACHABLE, currently Zephyr will ignore those (https://datatracker.ietf.org/doc/html/rfc4861#section-7.2.5)

To Reproduce

Just run IPv6 stack and observe packets on wireshark.

Expected behavior

Behavior compliant with RFC 4861.

Impact

May result in interoperability issues.