scripts: recover on partially downloaded files

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>

 #!/bin/sh

 SKIP_ME="### CT_NG SKIP ME ###"

 skip_me=$(($(cat "${0}" |@@grep@@ --binary-files=text -o -b -E "^${SKIP_ME}$" |cut -d : -f 1)+${#SKIP_ME}+1))

 dd if="${0}" bs=${skip_me} skip=1 2>/dev/null |bzcat

 exit 0

 ### CT_NG SKIP ME ###