give interfaces an if_bpf_mtap handler.

give interfaces an if_bpf_mtap handler.

the network stack is now responsible for calling bpf for packets
that the interface receives, and we so far got away with using
bpf_mtap_ether for everything. this doesn't work if layer 3 input
goes through the same functions, so letting drivers specify the
appropriate bpf mtap function means they will be able to cope.