TrafficMeter: ensure readability

As there are some devices without readability of '/proc/net/dev' (why? I can't get any idea), so let's get them back to the old method. Also, I'm afraid some devices' network interface does not start with 'eth' 'wlan' or 'rmnet', so I changed the logic into reading all interfaces excerpt 'lo'.
Merge pull request #390 from PeterCxy/patch-3

TrafficMeter: ensure readability