core/download fix local backend

core/download fix local backend

Since c8ef0c03b0b (download: put most of the infra in dl-wrapper), the
backend for local files is now named after the scheme, which is 'file'
for a local file.

>From the same commit on, the directory part and the basename are now
passed separately, to let the backend reconstruct the full path when it
needs to do so, which is the case for the 'file' backend too.

Finaly, ff559846fdc1 (support/download: Add support to pass options
directly to downloaders) introduced a nasty error, as it made use of
"${@}" when calling its internal function. Revert that mess now...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>