rtty support ip6 address

[leenzhu]

I'm happening to dealing with some systems with ip6 only network, I found rttys support ip6, but rtty doesn't.

After reading the rtty code, I made a little change, now rtty supports both ip4 and ip6 connection.

I'm not good at ip6, it just works. Hope this pull request would also help others that need ip6.

[leenzhu]

@zhaojh329 Any suggestions?

[zhaojh329]

Thanks. I'll review.