[feature request] Add support for permanent/hardware MAC address

[joaoantoniocardoso]

The current method, using cat /sys/class/net/$INTERFACE/address, returns the current MAC address, which can easily be changed.

The hardware MAC, also called permanent, however, is still available, and can be preferable in many applications.

Here's how ethtool and macchanger get the permanent MAC address, but since we are here using external tools, we could build a function to simply call ethtool -P $INTERFACE.

Thanks