NAT traversal: cleanup old functions

After the introduction of nat traversal code, there are some functions that can be removed (adapting the rest of the code).
The functions are:
- `new_networking()`
- `new_networking_ex()`
- `new_TCP_server()`

The solution is to rename `new_networking_nat()` to `new_networking()` and get rid of all its occurrences into the code; then doing the same for `new_TCP_server_nat()`.
