dhcpv6: create struct dhcpv6_lease

dhcpv6: create struct dhcpv6_lease

This concludes the lease/assignment struct saga. "struct dhcp_assignment" is
converted to "struct dhcpv6_lease" and various function names, etc are updated.

I intentionally did *not* change variable names here to keep the patch as
simple as possible. Therefore, the DHCPv6 codebase is still full of lots of
variables named things like "a". I expect that to change over time when I
tackle DHCPv6 code in ernest. But it'll have to do for now.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/292
Signed-off-by: Álvaro Fernández Rojas <[email protected]>