InfiniBand support

This is a follow-up to my Github-PR at https://github.com/isc-projects/kea/pull/134

Right now, Kea does not support serving to IPoIB clients, cause of the different framing which needs support by the bpf/lpf filters, as well as some slight differences here and there, mainly due to the huge Hardware-Addresses and non-fixed l2 broadcast address. The PR implements support for it on Linux via lpf, and should make it relatively straight forward to add it on the other platforms by someone who is in possession of the respective hardware.

I'd be happy to open the same PR here if that makes review easier, if given the permission to fork the Kea repository.