With this patch, we register a custom signal handler for the start
update subcommand that asks the user for confirmation if they trigger a
SIGINT signal, for example by pressing Ctrl+C.

This patch improves the error messages for the Nitrokey Start update
process so that it no longer recommends device reinsertion.  Instead,
users should try to repeat the update if it fails.  If they remove the
device after an unsuccessful update, they risk bricking the device.

With this patch, we replace the /releases API endpoint with
/releases/latest.  This endpoint only returns proper releases, not
pre-releases, so users are no longer offered firmware release
candidates.