For DLT_ATM_RFC1483, the network-layer header starts 8 bytes from the

For DLT_ATM_RFC1483, the network-layer header starts 8 bytes from the
beginning of the packet if the packet has an 802.2+SNAP header (3 bytes
802.2, 5 bytes SNAP), and 3 bytes from the beginning of the packet if it
has only an 802.2 header, just as is the case for DLT_ATM_CLIP, so go
back to handling them both with the same case.

Restore some comments asking whether we need to check the SSAP when
testing the 802.2 header for protocol types.

Clean up white space.

RFC 1188, RFC 1042, and RFCs 1483 and 2225 specify that SNAP
encapsulation is used for IP, not LLC encapsulation with LLCSAP_IP and,
in fact, that's what most if not all IP traffic over FDDI, 802 networks,
and LLC-encapsulated ATM use; go back to treating those link-layer types
the same way other link-layer types are handled.