Installer and luks support.

[Mathieu Othacehe]

Hi Guix,

I added support for disk encryption on the wip-newt-installer
branch. The user can select the guided partitioning scheme "Guided -
entire disk with encryption" or enable encryption partition by partition
in the manual partitioning mode.

I discovered an issue that also exist in other partitioning modes, and I
don't know how to deal with it. The last step of the installer consists
in:

* Mount partitions on /mnt (depending on the result of partitioning
  step).
* Write the system configuration to /mnt/etc/config.scm
* Start cow-store service
* Run guix system init /mnt/etc/config.scm /mnt
* Unmount /mnt
* Close luks mappings ---> fail, device is busy.

It fails because at this step, the overlayfs created by cow-store is
still present. To be able to close luks, I have to:

mkdir /remove-store-overlay
mount --move /gnu/store /remove-store-overlay
herd stop cow-store
herd stop guix-daemon
kill -9 <all processes spawned after starting cow-store>
umount /remove-store-overlay

Doing so, everything is cleaned-up, and it is possible to restart the
installer and run it again (in case the previous install failed or so).

Does anyone have a better idea on how to clean-up the overlayfs of
cow-store so that everything can be closed properly?

Thanks,

Mathieu