dhcp6-lease: Add function to fetch the IAID for the prefix

dhcp6-lease: Add function to fetch the IAID for the prefix

Add function to fetch the IAID for the delegated IA_PD prefix. In
order to keep things simple in the implemntation, the same IAID
is used with IA_NA addresses and IA_PD prefixes. But the DHCPv6
server can choose to return only IA_PD prefixes, and the client
can nowadays omit requesting of IA_NA addresses. Since the function
fetching said IAID from the lease looks only for IA_NA ones, it
will return an empty IAID, which of course does not match the one
set for prefixes.

Fix this by adding a function returning the IAID for the prefix.