all: implement RFC 5780 NAT Discovery

[ernado]

Maybe as "nat" subpackage.

[songjiayang]

Is it need stun server to help NAT discovery?

[ernado]

@songjiayang yes, the STUN Server need to support RFC 5780 too.
The gortcd currently does not support this.

About the general usage of NAT discovery:
If we can use ICE, the NAT discovery is not needed.

However, it can be used for diagnostics purposes.
E.g. some multiplayer games display NAT Status as "Strict" or "Open" to user.

[songjiayang]

Thanks for your explanation.

[ernado]

Closing in favour of separate client package.