Redo the way we map between libpcap encapsulation type codes and Wiretap

Redo the way we map between libpcap encapsulation type codes and Wiretap
encapsulation type codes - for those libpcap type codes whose numerical
value is interpreted differently by different versions of libpcap,
include <pcap.h> if you can and, if you can, use what it defines to
control which Wiretap code we map those type codes to.

Also, map the new libpcap type codes introduced by libpcap 0.5.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@2369 f5534014-38df-0310-8fa8-9805f1628bb7