Reuse a temp dir per device

[Dr-Emann]

Try to use the system tempdir first, then make a tempdir per device the root directories reside on.

This has several benefits:

• Most of the time, we'll use the common system tempdir entirely
• We will now exclude the tempdir we use for each root: it used to be possible for the temp files to be picked up by the reader thread: this is now only possible if we cross into a different device than all root dirs.