Merge pull request #31492 from yuwata/icmp6-packet

sd-ndisc: introduce ICMP6Packet and relevant functions, and use them

diff --cc src/libsystemd-network/ndisc-router-internal.h
Simple merge

diff --cc src/libsystemd-network/sd-ndisc-router.c
Simple merge

diff --cc src/network/networkd-ndisc.c
Simple merge