ipn: add AdvertiseRoutes to Prefs.

This is a prelude to supporting relaynode's --routes in
tailscaled. The daemon needs to remembers routes to
advertise, and the CLI needs to be able to change the
set of advertised routes. Prefs is the thing used for
both of these.

Signed-off-by: David Anderson <dave@natulte.net>

ipn/prefs_test.go

@@ -29,7 +29,7 @@ func checkPrefs(t *testing.T, p Prefs) {
 	}
 	p2 = p
 	p2.RouteAll = true
-	if p == p2 {
+	if p.Equals(&p2) {
 		t.Fatalf("p == p2\n")
 	}
 	p2b, err = PrefsFromBytes(p2.ToBytes(), false)