Download to an intermediate temp file, and rename it to its final
name only of download succeeds.
This catches both a failed download, and also the case where the user
interrupts the download. Thus, the a partial download gets discarded,
and we no longer try to extract a partial tarball, which we would
previously have done.
Suggested by Thomas PETAZZONI.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>