networkd-dhcp6: Assign DHCPv6 addresses and prefix lengths

Once IPv6 addresses have been acquired, assign these to the interface
with the prefix lengths taken from the ICMPv6 Router Advertisement
handling code. The preferred and valid IPv6 address lifetimes are
handed to the kernel which will clean up them if not renewed in time.

When a prefix announced via Router Advertisements expires, find all
addresses that match that prefix and update the address to have a
prefix length of 128 causing the prefix to be off-link.