MFC r258692 and r260207 (both by gnn).

r258692:
Add constants for use in interrogating various fiber and copper connectors
most often used with network interfaces.

The SFF-8472 standard defines the information that can be retrieved
from an optic or a copper cable plugged into a NIC, most often
referred to as SFP+.  Examples of values that can be read
include the cable vendor's name, part number, date of manufacture
as well as running data such as temperature, voltage and tx
and rx power.

Copious comments on how to use these values with an I2C interface
are given in the header file itself.

r260207:
Convert #defines to enums so that the values are visible in the debugger.