Provide packet type in afpacket

In order to detect the packet direction during capture it would be extremely helpful to provide the packet type as reported by the kernel via the `sll_pkttype` from the `sockaddr_ll` struct (as an optional feature). Since this is available directly in the C struct as received in afpacket it's fairly easy to retrieve and should have minimal to zero performance impact. In addition, extraction can be made optional via an option setter to avoid any impact if not required.