ipn, wgengine, magicsock, tsdns: be quieter and less aggressive when offline

If no interfaces are up, calm down and stop spamming so much. It was
noticed as especially bad on Windows, but probably was bad
everywhere. I just have the best network conditions testing on a
Windows VM.

Updates #604

wgengine/userspace.go

@@ -246,6 +246,7 @@ func newUserspaceEngineAdvanced(conf EngineConfig) (_ Engine, reterr error) {
 		e.tundev.Close()
 		return nil, fmt.Errorf("wgengine: %v", err)
 	}
+	e.magicConn.SetNetworkUp(e.linkState.AnyInterfaceUp())
 
 	// flags==0 because logf is already nested in another logger.
 	// The outer one can display the preferred log prefixes, etc.
@@ -1139,12 +1140,17 @@ func (e *userspaceEngine) LinkChange(isExpensive bool) {
 	cur.IsExpensive = isExpensive
 	needRebind, linkChangeCallback := e.setLinkState(cur)
 
-	if needRebind {
-		e.logf("LinkChange: major, rebinding. New state: %+v", cur)
+	up := cur.AnyInterfaceUp()
+	if !up {
+		e.logf("LinkChange: all links down; pausing: %v", cur)
+	} else if needRebind {
+		e.logf("LinkChange: major, rebinding. New state: %v", cur)
 	} else {
 		e.logf("LinkChange: minor")
 	}
 
+	e.magicConn.SetNetworkUp(up)
+
 	why := "link-change-minor"
 	if needRebind {
 		why = "link-change-major"