wgengine: add Engine.SetLinkChangeCallback

Start of making the IPN state machine react to link changes and down
its DNS & routes if necessary to unblock proxy resolution (e.g. for
transitioning from public to corp networks where the corp network has
mandatory proxies and WPAD PAC files that can't be resolved while
using the DNS/routes configured previously)

This change should be a no-op. Just some callback plumbing.

ipn/local.go

@@ -19,6 +19,7 @@ import (
 	"tailscale.com/internal/deepprint"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/ipn/policy"
+	"tailscale.com/net/interfaces"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/portlist"
 	"tailscale.com/tailcfg"
@@ -109,11 +110,23 @@ func NewLocalBackend(logf logger.Logf, logid string, store StateStore, e wgengin
 		state:        NoState,
 		portpoll:     portpoll,
 	}
+	e.SetLinkChangeCallback(b.linkChange)
 	b.statusChanged = sync.NewCond(&b.statusLock)
 
 	return b, nil
 }
 
+func (b *LocalBackend) linkChange(major bool, ifst *interfaces.State) {
+	// TODO(bradfitz): on a major link change, ask controlclient
+	// whether its host (e.g. login.tailscale.com) is reachable.
+	// If not, down the world and poll for a bit. Windows' WinHTTP
+	// service might be unable to resolve its WPAD PAC URL if we
+	// have DNS/routes configured. So we need to remove that DNS
+	// and those routes to let it figure out its proxy
+	// settings. Once it's back up and happy, then we can resume
+	// and our connection to the control server would work again.
+}
+
 // Shutdown halts the backend and all its sub-components. The backend
 // can no longer be used after Shutdown returns.
 func (b *LocalBackend) Shutdown() {