Added the SetBPFFilter function to afpacket.

To make this possible, I had to add the OpenDead()
function to the pcap package and also to have the
BPF type able to export the compile program. This
also means that afpacket depends on the pcap package
now.
We need SOL_SOCKET not SOL_PACKET.

Also, make the error messages more useful by indicating which
function failed.
Merged bpf_for_afpacket