pkg-download: handle interrupted wget downloads

When a wget download is interrupted, the downloaded file is still created.
It will therefore not be re-downloaded in the next build, and the
extraction will fail.

To avoid this, download to a temporary file first and rename when the
download is successful.

The existing mechanism doesn't work for interrupted downloads because the
whole sub-shell is interrupted, so the rm-part never gets executed.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>