Brad Fitzpatrick e3df29d488 wgengine{,/monitor}: move interface state fetching/comparing to monitor
Gets it out of wgengine so the Engine isn't responsible for being a
callback registration hub for it.

This also removes the Engine.LinkChange method, as it's no longer
necessary.  The monitor tells us about changes; it doesn't seem to
need any help. (Currently it was only used by Swift, but as of
14dc79013754fe we just do the same from Go)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
2021-03-01 13:01:08 -08:00
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