GWinfo: batX has to be assigned using MAC, not netif name

[adschm]

This is a reminder for myself: If we assign bat to vpn by name, every restart will generate new devices with new MAC addresses, but the same name. Thus, we have two options:

1. We evaluate netif name to MAC address when parsing the GWinfo data and save the MAC
2. We delete old MACs before adding new ones

[adschm]

Done with solution 1.