ZFS package refactor

[Kaelyn]

Hi,

I have just rebased my patch for splitting the ZFS package into separate 
user-space tool and kernel module packages, to resolve the merge conflict from 
the current ZFS package being updated to a newer version. The patch can be 
found at https://issues.guix.gnu.org/71482.

Among other things means that neither the default linux-libre kernel  nor the 
ZFS kernel module for that kernel have to be built when one is using a 
different kernel package. This is particularly noticeable if one references the 
existing zfs package for the sake of having the tools available. Currently one 
has to be careful to ensure that only the custom zfs package derivative is 
referenced in the list of operating-system packages and any services or other 
packages that might depend on the user-space tools (such as zfs-auto-snapshot) 
to avoid having to download and/or build a kernel and matching module that 
isn't used.

Cheers,
Kaelyn