support/download: add option parsing to the download wrapper

Instead of relying on argument ordering, use actual options in the
download wrapper.

Download backends (bzr, cp, hg...) are left as-is, because it does not
make sense to complexify them, since they are almost very trivial shell
scripts, and adding option parsing would be really overkill.

This commit also renames the script to dl-wrapper so it looks better in
the traces, and it is not confused with another wrapper.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>