Add "ipx", which checks for the LLC SAP for IPX as well as, on Ethernet,

Add "ipx", which checks for the LLC SAP for IPX as well as, on Ethernet,
for "Novell 802.3" frames, which are 802.3 frames (i.e., the type/length
field is a length field, i.e. it's <= ETHERMTU) with 0xFFFF as the first
2 bytes.  We don't yet check for ETHERTYPE_IPX as well.

When checking for OSI packets on Linux cooked captures, check for 802.2
frames by testing the packet type for LINUX_SLL_P_802_2 rather than by
checking whether the type field is <= ETHERMTU (it's always a type field
in DLT_LINUX_SLL captures).