all: use parallel dialer for bootstrapping ip

So we don't have to depend on network probing for checking ipv4/ipv6
enabled, making ctrld working more stably.

cmd/ctrld/os_linux.go

@@ -112,7 +112,7 @@ func resetDNS(iface *net.Interface) (err error) {
 	}
 
 	// TODO(cuonglm): handle DHCPv6 properly.
-	if ctrldnet.SupportsIPv6() {
+	if ctrldnet.IPv6Available(ctx) {
 		c := client6.NewClient()
 		conversation, err := c.Exchange(iface.Name)
 		if err != nil {

cmd/ctrld/prog.go

@@ -74,7 +74,7 @@ func (p *prog) run() {
 		uc.Init()
 		if uc.BootstrapIP == "" {
 			uc.SetupBootstrapIP()
-			mainLog.Info().Str("bootstrap_ip", uc.BootstrapIP).Msgf("Setting bootstrap IP for upstream.%s", n)
+			mainLog.Info().Msgf("Bootstrap IPs for upstream.%s: %q", n, uc.BootstrapIPs())
 		} else {
 			mainLog.Info().Str("bootstrap_ip", uc.BootstrapIP).Msgf("Using bootstrap IP for upstream.%s", n)
 		}