types/persist: split controlclient.Persist into a small leaf package

This one alone doesn't modify the global dependency map much
(depaware.txt if anything looks slightly worse), but it leave
controlclient as only containing NetworkMap:

bradfitz@tsdev:~/src/tailscale.com/ipn$ grep -F "controlclient." *.go
backend.go:     NetMap        *controlclient.NetworkMap // new netmap received
fake_test.go:   b.notify(Notify{NetMap: &controlclient.NetworkMap{}})
fake_test.go:   b.notify(Notify{NetMap: &controlclient.NetworkMap{}})
handle.go:      netmapCache       *controlclient.NetworkMap
handle.go:func (h *Handle) NetMap() *controlclient.NetworkMap {

Once that goes into a leaf package, then ipn doesn't depend on
controlclient at all, and then the client gets smaller.

Updates #1278

ipn/prefs_clone.go

@@ -8,8 +8,8 @@ package ipn
 
 import (
 	"inet.af/netaddr"
-	"tailscale.com/control/controlclient"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/persist"
 	"tailscale.com/types/preftype"
 )
 
@@ -24,7 +24,7 @@ func (src *Prefs) Clone() *Prefs {
 	dst.AdvertiseTags = append(src.AdvertiseTags[:0:0], src.AdvertiseTags...)
 	dst.AdvertiseRoutes = append(src.AdvertiseRoutes[:0:0], src.AdvertiseRoutes...)
 	if dst.Persist != nil {
-		dst.Persist = new(controlclient.Persist)
+		dst.Persist = new(persist.Persist)
 		*dst.Persist = *src.Persist
 	}
 	return dst
@@ -50,5 +50,5 @@ var _PrefsNeedsRegeneration = Prefs(struct {
 	AdvertiseRoutes  []netaddr.IPPrefix
 	NoSNAT           bool
 	NetfilterMode    preftype.NetfilterMode
-	Persist          *controlclient.Persist
+	Persist          *persist.Persist
 }{})