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Provide new KPI for network drivers to access lists of interface

Description

Provide new KPI for network drivers to access lists of interface
addresses. The KPI doesn't reveal neither how addresses are stored,
how the access to them is synchronized, neither reveal struct ifaddr
and struct ifmaddr.

Reviewed by: gallatin, erj, hselasky, philip, stevek
Differential Revision: https://reviews.freebsd.org/D21943