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.

wgengine/watchdog.go

@@ -16,6 +16,7 @@ import (
 	"inet.af/netaddr"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/net/interfaces"
 	"tailscale.com/tailcfg"
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/router"
@@ -100,6 +101,9 @@ func (e *watchdogEngine) RequestStatus() {
 func (e *watchdogEngine) LinkChange(isExpensive bool) {
 	e.watchdog("LinkChange", func() { e.wrap.LinkChange(isExpensive) })
 }
+func (e *watchdogEngine) SetLinkChangeCallback(cb func(major bool, newState *interfaces.State)) {
+	e.watchdog("SetLinkChangeCallback", func() { e.wrap.SetLinkChangeCallback(cb) })
+}
 func (e *watchdogEngine) SetDERPMap(m *tailcfg.DERPMap) {
 	e.watchdog("SetDERPMap", func() { e.wrap.SetDERPMap(m) })
 }