[bug#45692] [PATCH 0/4] Even Better ZFS Support on Guix

[raid5atemyhomework]

Hi all,

Sorry for the excessive number of patches.

This patchset contains 4 patches, two that add new features to Guix and do not 
involve ZFS at all, one bugfix of the ZFS package, and one which makes 
installing ZFS on Guix easy.

* Patch 1: Create an extensible service type that installs kernel-loadable 
modules into the kernel.
  * This allows a ZFS service to add the ZFS module to the kernel.
* Patch 2: Make the `file-systems` shepherd service an extensible target, so 
that other services can add more requirements for `file-systems`.
  * This allows a ZFS service to make `file-systems` wait for the ZFS automount 
at startup, which is necessary in any case in order to ensure that ZFS has 
scanned for ZFS pools installed in the hardware.
* Patch 3: Fix the ZFS package definition.
* Patch 4: Add a ZFS service type that installs everything needed for basic ZFS 
usage!

I've tested the ZFS functionality in about a half-dozen VMs already, including 
checking that the ZFS setup persists across reboots of the VM:

* No ZFS installed.
* With ZFS installed but without any ZFS pools.
* With ZFS pool that is automounted.
* With ZFS pool in legacy mode (needs manual `mount -t zfs poolname 
/mountpoint`).
* Encrypted ZFS dataset (prompts passphrase at boot to mount).
* Encrypted ZFS dataseet as `/home` (prompts passphrase at boot to mount, user 
`/home` directories are created and maintained).

Other bits of ZFS functionality would be *nice* but aren't in this patchset yet 
(and at least for my purposes are not really necessary, except perhaps the 
auto-scrubbing, I just need to wrap my head around `mcron`...):

* Auto-scrubbing of ZFS pools to ensure no data corruption.
* Auto-snapshotting of ZFS datasets to protect against user error.
* `/` on ZFS.
* `/boot` on ZFS.
* Managing ZFS "legacy" mountpoints on  `(file-system ...)` declarations.
* Managing ZFS pool setup in the `operating-system` declaration somehow (not 
sure how to do this).

Thanks
raid5atemyhomework