There were many instances of an ifdef idiom checking for one of many
Unix-like systems, and an else for Windows.  This idiom fails to build
on a new Unix-like system and needs significant edits.

Replace most instances of the list of systems with a "if this system
is not windows" and an else (for windows), intending to have no code
change on any system in the prior list, and extending portability to
Unix-like systems not in the prior list.

Another idiom is "if this is windows", followed by an else or if for a
list of Unix-like systems.  Replace this with a simple "if windows
else".

As part of this cleanup, in several places, an else clause -- for
systems that aren't in the list and aren't windows -- was removed.
This never made sense and is a bugfix.

In the one case where there is a correct "if linux or freebsd"
(ethernet.h include), explain why it's there and implicitly why it's
not a pattern to be extended to every new system.