Disable Wake on lan when offline. Adds `ethtool` dependency on the package, however…

Hey, 

I've added another setting to `ftw`: Disable Wake on lan when on battery. Generally, this could be off for me all the time, but I guess that wouldn't be right to set.

However, this option adds a new dependency to the package: **ethtool**.

So yeah, I don't know whether you wanna have this!?