Refactor Prefix Delegation configuration so we use integers instead

Refactor Prefix Delegation configuration so we use integers instead
of hex based arrays.
The SLA can now be applied to any router given prefix length provided the
SLA can fit into the prefix and desired prefix length.
We now wait for a carrier and LL address before adding the addresses from the
delegated prefix. Before adding this code, I added some code to generate a
LL address from the hardware address. This is now #ifdef'ed out as I don't
want to throw it away in-case dhcpcd ever needs to create LL addresses
itself.

Many thanks to Martin Husemann who developed code for these functions:
	ipv6_userprefix, in6_to_h64 and h64_to_in6
which are used to generate a prefix from the delegated prefix and the SLA