Many changes in register_camera.sh and footage_downloader.sh

Use register_camera.sh to associate multiplexer id / ssd id pairs with
ssd serials or camera module numbers, so that no confusion is possible
when requesting connection from both multiplexers simultaneously in
footage_downloader.sh (should be run once for all (a few times to be sure)
then stored eg in /data/camera/<MAC>/info)

In the current footage_dowloader.sh, mux requests are still sequential.

Script line numbers are now displayed in the log.

fsck -p is safer (TODO: implement device copy with dd when fsck -y is needed)

Cleaner exit (ctrl-c exits gracefully), exit code after "wait" is run.

Removed scsi devices are added on exit

...