make tox use loopback device on linux when UDP and local lan is turned on

[zoff99]

now tox is not using lo: (loopback) 127.0.0.1 to connect to devices on the same host. even when UDP and local lan is turned on.
now tox will only listen on the eth0: device (or whatever your LAN device is called)
loopback can get much higher transfer speeds than LAN.

this shows a hackish PoC how to enable 127.0.0.1
https://gist.github.com/zoff99/f8d605514a89d995c526bd6577a49051
(but this ONLY uses 127.0.0.1 and no other network device)

we should add an enhancement to detect local loopback device and make tox also listen on that for connections.
and probably the same for the other direction (trying to connect to a node on local loopback)