the real pcap_inject in the case where we do have it.

libpcap are lacking the function.  For now, we always use the
substitute version, since there's no easy way to determine whether or
not we need it.

Include automatic filtering (based on source MAC address of first
transmitted packet).

Proven to successfully elicit a ping response from a remote host.

socket, accept new connections, open a network interface using libpcap
and forward (all) traffic.