all: rework bootstrap IP discovering

At startup, ctrld gathers bootstrap IP information and use this
bootstrap IP for connecting to upstream. However, in case the network
stack changed, for example, dues to VPN connection, ctrld will still use
this old (maybe invalid) bootstrap IP for the current network stack.

This commit rework the discovering process, and re-initializing the
bootstrap IP if connecting to upstream failed.

resolver.go

@@ -93,5 +93,8 @@ func (r *legacyResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, e
 		Dialer: dialer,
 	}
 	answer, _, err := dnsClient.ExchangeContext(ctx, msg, r.endpoint)
+	if _, ok := err.(*net.OpError); ok {
+		return answer, ErrUpstreamFailed
+	}
 	return answer, err
 }