re-implement netmod-inet

The previous netmod-inet code got us to this point but it's not really maintainable anymore. It had bugs that were hard to fix, was too complicated, and in my opinion made some bad assumptions. Furthermore it was hard to add any features to it.

This new implementation will have a slightly different design goals

• "Standard" connections are client-server
• "Cross" connections are server-server
• Support multi-bind (IPv6 and IPv4)
• Support STUN for NAT traversal instead of relying on UPnP
• Support two-stage auto discovery
• Add a section in the manual for how to configure inet

The new module doesn't do any of these things but it's more predictable and the code is cleaner. I'll try adding some docs to the developer manual as to how it works so that we can avoid having to re-write it again in the future :')