Use the correct gettext domain in shutdown-helper.
Add a special case for shutdown-helper's temporary pot file.

Quite simply, "extension.js.pot" isn't unique enough.
Set group ownership to floppy only when devices are added.
Only make floppy own devices that we support installing Tails to.

Tails Installer requires raw block device access to such devices,
which is why we have this workaround in the first place.
Fix a race between reboot/halt and the remote shell.

When e.g. `halt` is called with `execute` we wait for the remote shell
to respond, which it won't if the remote shell is killed fast
enough. This seems to be the case sometimes in Jessie (thanks to the
use of systemd) so let's use `spawn` instead.
Update some more test suite images for Jessie.