Prevent using root as mount point for external storage

Fixes #5981

Currently there is no way to display a message, so this approach will only safeguard setting root as mountpoint and getting unexpected results.

I tried to write unit tests and cover a bit more but it seems that mocking/stubbing static classes don't work properly or are close to impossible, so I give up for now... The unit tests are minimal and only test for the "check root mount point" condition for now.

Please review @DeepDiver1975 @karlitschek @icewind1991 @schiesbn