cmd/tailscale/cli: plumb TUN mode into tailscale web template

UI works remains, but data is there now.

Updates #4015

Change-Id: Ib91e94718b655ad60a63596e59468f3b3b102306
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -579,6 +579,7 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 	defer b.mu.Unlock()
 	sb.MutateStatus(func(s *ipnstate.Status) {
 		s.Version = version.Long
+		s.TUN = !wgengine.IsNetstack(b.e)
 		s.BackendState = b.state.String()
 		s.AuthURL = b.authURLSticky
 		if err := health.OverallError(); err != nil {

ipn/ipnstate/ipnstate.go

@@ -31,6 +31,10 @@ type Status struct {
 	// Version is the daemon's long version (see version.Long).
 	Version string
 
+	// TUN is whether /dev/net/tun (or equivalent kernel interface) is being
+	// used. If false, it's running in userspace mode.
+	TUN bool
+
 	// BackendState is an ipn.State string value:
 	//  "NoState", "NeedsLogin", "NeedsMachineAuth", "Stopped",
 	//  "Starting", "Running".