xfer: Improve how we create temporary files

xfer: Improve how we create temporary files

We will now create invisible files in the destination file system which
gives us the advantage that the file will be written where it should be
and we won't have to move it later.

We can simply link the insivible file to become visible in the file
system on success. Otherwise we just close the file handle and the data
will be freed again.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>