I've seen captures with all four different flavors of IPX frames on

I've seen captures with all four different flavors of IPX frames on
Ethernet, so, at least on Ethernet, when checking for IPX frames, check
for all of them, including Ethernet_II and Ethernet_SNAP.

Add an "llc.h" file with LLC SAP values, taken from tcpdump's "llc.h"
file, and use those, rather than defining them ourselves in "gencode.c".