When deleting an encryption key, check if there is at least one other
active keyslot, and do not delete if there isn't.

Replace the very similar encryption_keys_post and encryption_keys_put
callback functions by a single handler function, with a boolean flag to
separate the two variants.

basic PUT and DELETE behavior, still does some weird stuff with random unicode characters appearing some times

In case the device is unencrypted, check whether encryption seems
possible (depending on filesystem usage and available memory), and
report that in the response.

Sleep for a bit after we received a signal to stop cryptops-api. This
gives the request handler that asked to stop the server more time to
send its response to the client, fixing a race condition.

Extend the `encryption/add` command to repartition the encrypted container,
copy the authorized_keys file to the configuration partition, generate a new
ssh host key for the dropbear running in the initrd, and reboot after it's
done.