Correctly detect when MAC address was not spoofed on NICs that macchanger cannot get the permanent MAC of

Details are in the report sent by intrigeri to tails-dev on 2013-12-27.

As anonym put it, we’d need to save the MAC address to somewhere in the
filesystem in tails-spoof-mac (called by the MAC spoof udev hook)
since we also need the functionality of nic_has_spoofed_mac() in the
fails-safe in tails-unblock-network.

Feature Branch: feature/spoof-mac

Parent Task: #5421 (closed)

Original created by @intrigeri on 6552 (Redmine)