Fixed reading the MAC address on Linux systems directly from /sys/class/net/*/address

This didn't work until now, because the statement `path and File.read(…)` always evaluated to `nil` as the variable `path` simply didn't exist.

This means, Facter **always** used `ifconfig` to determine the MAC address which is slow, can't be parsed reliably, outdated and unmaintained.

I'll work on replacing the `ifconfig` usage in Facter with `iproute2` where it makes sense or reading the required information directly from `/sys` files etc.