Try to detect/warn in greeter if user has plugged tails device into untrusted system
See https://mailman.boum.org/pipermail/tails-dev/2016-February/010302.html, and the wider thread for background info.
In summary, some users are plugging in their Tails USB device into non-trustworthy systems, (e.g., Windows / OSX), and potentially infecting themselves. This is often noticeable because of stray files these OSes leave behind, e.g., .DS_Store, Thumbs.db, __MACOSX, etc.
We should detect these files and if any are found on the device, warn the user that their device is potentially compromised. Until #7496 is finished, we don’t have any way to verify the Tails installation. Advising the user to make a clean install is likely the best course of action at this point.
A further improvement would be to detect hidden partitions on the device, and potentially change the warning as appropriate.
- Related to #11102