homework: upload home password into kernel keyring if needed

homework: upload home password into kernel keyring if needed

If we do automatic disk space rebalancing, we must be able to unlock the
encrypted volume for that in the background, thus we need to decryption
key around in userspace. Let's do this via the kernel keyring. This
allows us to do this in a relatively secure way, so that it sticks
around between homework invocations, but still is destroyed
automatically when homed goes down.