On many VPS providers, the IPv4 uplink network interface is eth0, while the IPv6 uplink may be on a different interface (e.g., eth1 or another device).

This PR:

• Improves automatic detection of the uplink network interface for both IPv4 and IPv6.
• Adds support for configurations where the IPv4 and IPv6 uplinks are on different interfaces, while remaining fully compatible with setups where they share the same interface.
• Changes the expect environment variable name from NETWORK_DEVICE to NET4_DEVICE and NET6_DEVICE

This change is