ipn/ipnlocal,net/netmon: make frequent darkwake more efficient

Investigating battery costs on a busy tailnet I noticed a large number
of nodes regularly reconnecting to control and DERP. In one case I was
able to analyze closely `pmset` reported the every-minute wake-ups being
triggered by bluetooth. The node was by side effect reconnecting to
control constantly, and this was at times visible to peers as well.

Three changes here improve the situation:
- Short time jumps (less than 10 minutes) no longer produce "major
  network change" events, and so do not trigger full rebind/reconnect.
- Many "incidental" fields on interfaces are ignored, like MTU, flags
  and so on - if the route is still good, the rest should be manageable.
- Additional log output will provide more detail about the cause of
  major network change events.

Updates #3363

Signed-off-by: James Tucker <james@tailscale.com>

ipn/ipnlocal/local.go

@@ -582,7 +582,7 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 
 	// Call our linkChange code once with the current state.
 	// Following changes are triggered via the eventbus.
-	cd, err := netmon.NewChangeDelta(nil, b.interfaceState, false, false)
+	cd, err := netmon.NewChangeDelta(nil, b.interfaceState, 0, false)
 	if err != nil {
 		b.logf("[unexpected] setting initial netmon state failed: %v", err)
 	} else {