Add captured direction in pcap_pkthdr

[teknoraver]

No description provided.

[guyharris]

This changes a structure in ways that break the ABI; programs using pcap_next() that were built with a version of libpcap without the added member of struct pcap_pkthdr will, if they call pcap_next() when running with a version of libpcap with that member, get data following the structure overwritten.

[teknoraver]

I need this feature to show the direction in tcpdump:

the-tcpdump-group/tcpdump#678

How can we do it without breaking the ABI?

[teknoraver]

I solved it in a different way, I allow to force a cooked capture as the datalink headers often provides this information.

Please see #784