Randomise MAC address when scanning for Wi-Fi networks even when MAC spoofing is disabled

Apparently, Apple is going to introduce that: https://twitter.com/lmjabreu/status/475594066907111424/photo/1.
Of course, they’re going to spoof MAC only for proble requests, not when actually connecting to an AP.

It might be good for Tails to do that when MAC spoofing opted-out from in the Greeter: then, you reveal your real MAC address to the AP you actually connect to, but not to others. This way, users get the benefit of not spoofing, when they need to disable it (e.g. to connect to a filtering AP), but without the drawback of broadcasting their real MAC address around.

Related issues

• Related to #6453

Original created by @intrigeri on 7380 (Redmine)