Narrow the BPF read filter rules so only packets sent to the

Narrow the BPF read filter rules so only packets sent to the
interface's LLADDR pass. Rely on dhclient's existing ability to
detect and react to LLADDR changes.

This limits the number of packets that get dropped as a result of
dhclient setting BIOCSFILDROP on the bpf descriptor.

Problem with bridges and multiple dhclients noted by stsp@.

ok mpi@ stsp@ deraadt@ henning@