sd-dhcp6-client: Receive and parse Advertise messages

When receiving DHCPv6 messages, discard the ones that are not meant
for DHCPv6 clients and verify the transaction id. Once that is done,
process the Advertise message and select the Advertise with the
highest preference.

Create a separate function for lease information parsing so that it
can be reused in other parts of the protocol. Verify both DUID and
IAID in the received message and store other necessary information
with the lease structure.