[netlink] Convert interfaces to Netlink links

Convert from Netstack interface properties structs to Netlink Link
messages. This is a set-up in preparation for sending these interface
updates to RTMGRP_LINK when an interface watcher event has an
online/offline change.

Tests included for loopback/non-loopback interfaces.

Bug: https://issuetracker.google.com/283137644
Change-Id: I9939af82646c0040d5c2fb3d85ff8568d07e95be
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/862360
Reviewed-by: Jeff Martin <martinjeffrey@google.com>
Reviewed-by: Ghanan Gowripalan <ghanan@google.com>
Fuchsia-Auto-Submit: Antonio Linhart <antoniolinhart@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>