net/interfaces, wgengine/monitor: fix false positives link changes

interfaces.State.String tries to print a concise summary of the
network state, removing any interfaces that don't have any or any
interesting IP addresses. On macOS and iOS, for instance, there are a
ton of misc things.

But the link monitor based its are-there-changes decision on
interfaces.State.Equal, which just used reflect.DeepEqual, including
comparing all the boring interfaces. On macOS, when turning wifi on or off, there
are a ton of misc boring interface changes, resulting in hitting an earlier
check I'd added on suspicion this was happening:

    [unexpected] network state changed, but stringification didn't

This fixes that by instead adding a new
interfaces.State.RemoveUninterestingInterfacesAndAddresses method that
does, uh, that. Then use that in the monitor. So then when Equal is
used later, it's DeepEqualing the already-cleaned version with only
interesting interfaces.

This makes cmd/tailscaled debug --monitor much less noisy.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/monitor/monitor.go

@@ -100,6 +100,7 @@ func (m *Mon) interfaceStateUncached() (*interfaces.State, error) {
 	s, err := interfaces.GetState()
 	if s != nil {
 		s.RemoveTailscaleInterfaces()
+		s.RemoveUninterestingInterfacesAndAddresses()
 	}
 	return s, err
 }