ZeroTier transport changes

[janjaapbos]

Fix for:

• zt_ep_set_add_local_addr
• zt_ep_set_clear_local_addrs

New methods to retrieve udp address used for ZeroTier:

• zt_ep_get_udp4_addr
• zt_ep_get_udp6_addr

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[gdamore]

I know things have changed a lot.

I still plan to revisit ZT in general. I'm convinced that 6PLANE is the way to go in the long run.

However, in the meantime I need to look at more modest changes like this.

Thanks for your contribution -- I will be following up.

[janjaapbos]

I will do rewrite of these methods for setting and getting udp address, taking into account the comments above.

[dch]

FWIW I use ZT 6PLANE to link up our containers across servers and different providers. It's so so sweet to use. @janjaapbos it would be great to see some more ZT+NNG code examples about the place to look through, if you have some to share.