- We now have a single tmpdir "root" that can be recreated at launch to
clean up old stragglers
- The location of this tmpdir root can be controlled by `XTL_TMPDIR` or
`TMPDIR`. In general the path is `$TMPDIR/sh.xtool`.
With this change it should be possible to `export XTL_TMPDIR=/var/tmp`
if `/tmp` doesn't have enough space, which fixes#23.
This was vestigial, and besides the fact that we don't actually do any
work in this stage, we also never update the progress here so it gets
stuck at 0%.
- Always try to register the device and just return gracefully if it
fails (prevents races, pagination issues)
- Allow `SigningContext` (and therefore `AutoSigner`) to be created
without a target device: just skip device registration.
