cmd/tailscale/main.go: use ipn.NewPrefs(), not &ipn.Prefs{}

ipn.NewPrefs() sets accepting DNS and routes to true,
among other things. &ipn.Prefs{} initializes all fields
to false.

Fixes https://github.com/tailscale/corp/issues/13377

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

cmd/tailscale/main.go

@@ -357,7 +357,8 @@ func (a *App) runBackend() error {
 			}
 			first := state.Prefs == nil
 			if first {
-				state.Prefs = &ipn.Prefs{Hostname: a.hostname()}
+				state.Prefs = ipn.NewPrefs()
+				state.Prefs.Hostname = a.hostname()
 				go b.backend.SetPrefs(state.Prefs)
 				a.setPrefs(state.Prefs)
 			}