This splits out a bunch of functions from fileio.c that have to do with
temporary files. Simply to make the header files a bit shorter, and to
group things more nicely.
No code changes, just some rearranging of source files.
Let's handle tarball imports nicer that have a single top-level
directory containing the OS tree: let's move everything down during
import, so that the OS tree is ready to use automatically.
Fixes: #2116
Fixes: #2728
This is also supposed to be preparation for doing #10234 eventually,
where a very similar operation is requested: instead of importing a tree
to /var/lib/machines it would need to be imported into
/var/lib/portables/.
