Handle the three types of link layers:

Handle the three types of link layers:

	those that always use 802.2;

	those that never use 802.2;

	Ethernet (where 802.2 is used for 802.3 and is not used for
	Ethernet II);

correctly.  This requires having two variables for the offset of the
network layer header, one for use with protocols that would not run atop
802.2 on Ethernet and would run atop 802.2+SNAP on link layers that
always use 802.2, and one for use with protosol that would run atop
raw 802.2 (no SNAP) on Ethernet and on link layers that always use
802.2.

Fix the network layer offset for 802.11, and the link-layer offset for
RFC 1483 ATM (there's no link layer, there's just 802.2 LLC).