ipv6: request IA_PD prefix for DHCP-PD

This fixes issue #447. I would still consider this patch set **highly experimental** due to how particular some IPv6 setups seem to be. In theory this should not affect anything other than DHCP-PD users, but I will need feedback from non-PD, static and tunnel users.

I have tested on Comcast with DHCP-PD and I am able to acquire a /60 prefix with a /64 subnet assigned to the LAN interface.

Currently I have hardcoded a request of a /56. This is somewhat arbitrarily chosen mostly based off the D-Link DIR-655 boot log in [this post on DSLReports](http://www.dslreports.com/forum/r28736022-). The DHCPv6 server is free to ignore the requested prefix and assign whatever prefix it wants anyway, so this should not be any issue.