icmp: clean up the extension code and comments.

icmp: clean up the extension code and comments.

Remove remaining references to them as being MPLS-related;
draft-bonica-icmp-mpls-02 referred to them as "ICMP Extensions for
MultiProtocol Label Switching", but draft-bonica-internet-icmp-00
redefined it as a general extension mechanism, not MPLS-related, and
that draft eventually became RFC 4884.

draft-bonica-icmp-mpls-02 also defined class 2 objects as objects
containing the remaining packet information past the first 128 bytes;
that never became part of an RFC and class 2 was redefined as an
"Interface Information Object" in RFC 5837.

Give some of these details in comments, and move some
extension-related definitions among the rest of the extension stuff.

(backported from commit 6d3b0e4599c5aa922bba42e53b038b51d9024a62)