
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
14dc790137
we just do the same from Go)
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>