Merge pull request #7582 from pfl/dhcp6_prefix_delegation

DHCPv6 prefix delegation

diff --cc man/systemd.network.xml
Simple merge

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

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

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

diff --cc src/network/networkd-manager.h
Simple merge