Use if_link instead of the alias if_list, and change a for() into the TAILQ_FOREACH() form.
Use if_link instead of the alias if_list, and change a for() into
the TAILQ_FOREACH() form.

Comment the need to store the same info (mac address for ethernet-type
devices) in two different places.

No functional changes. Even the compiler output should be unmodified
by this change.

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