zfsutils-linux fails to install if `modprobe zfs` fails

== SRU Justification, Focal ==

Justification: Installing zfsutils with zfs modules not available causes the package to installation to fail. The package should install cleanly rather than break.

== Test Case ==

# Prevent module loading
$ sudo sysctl kernel.modules_disabled=1

# Install the package
$ sudo apt install -y zfsutils-linux

This should succeed. Without the fix this fails. (See below for the failure message). With the fix modules don't install but the package install succeeds.

== Regression Potential ==

There may be users who expect the package to fail to install when the zfs utils fail to install when the modules are not available. However, this is a corner case where users are expecting broken behaviour to occur and should not be relying on this.

The changes to the systemd rules are already in Ubuntu groovy and have been tested, so I believe the regression potential is limited.

----

Description:

When installing zfsutils-linux, failure to `modprobe zfs` cause the package installation to fail.
Ideally, modprobe should be attempted but not fatal.

Steps to reproduce:

# Prevent module loading
$ sudo sysctl kernel.modules_disabled=1

# Install the package
$ sudo apt install -y zfsutils-linux

This should succeed but fails with:

Reading package lists... Done
Building dependency tree
Reading state information... Done
zfsutils-linux is already the newest version (0.8.3-1ubuntu12).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
2 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Setting up zfsutils-linux (0.8.3-1ubuntu12) ...
insmod /lib/modules/5.4.0-31-generic/kernel/zfs/zlua.ko
modprobe: ERROR: could not insert 'zfs': Operation not permitted
zfs-import-scan.service is a disabled or a static unit, not starting it.
A dependency job for zfs-import-cache.service failed. See 'journalctl -xe' for details.
Job for zfs-load-module.service failed because the control process exited with error code.
See "systemctl status zfs-load-module.service" and "journalctl -xe" for details.
zfs-import-scan.service is a disabled or a static unit, not starting it.
Job for zfs-load-module.service failed because the control process exited with error code.
See "systemctl status zfs-load-module.service" and "journalctl -xe" for details.
A dependency job for zfs-import-cache.service failed. See 'journalctl -xe' for details.
Job for zfs-share.service failed because the control process exited with error code.
See "systemctl status zfs-share.service" and "journalctl -xe" for details.
invoke-rc.d: initscript zfs-share, action "start" failed.
● zfs-share.service - ZFS file system shares
     Loaded: loaded (/lib/systemd/system/zfs-share.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Sun 2020-05-24 16:56:03 UTC; 52ms ago
       Docs: man:zfs(8)
    Process: 9283 ExecStartPre=/bin/rm -f /etc/dfs/sharetab (code=exited, status=0/SUCCESS)
    Process: 9284 ExecStart=/sbin/zfs share -a (code=exited, status=1/FAILURE)
   Main PID: 9284 (code=exited, status=1/FAILURE)

May 24 16:56:03 ocelot systemd[1]: Starting ZFS file system shares...
May 24 16:56:03 ocelot zfs[9284]: The ZFS modules are not loaded.
May 24 16:56:03 ocelot zfs[9284]: Try running '/sbin/modprobe zfs' as root to load them.
May 24 16:56:03 ocelot systemd[1]: zfs-share.service: Main process exited, code=exited, status=1/FAILURE
May 24 16:56:03 ocelot systemd[1]: zfs-share.service: Failed with result 'exit-code'.
May 24 16:56:03 ocelot systemd[1]: Failed to start ZFS file system shares.
dpkg: error processing package zfsutils-linux (--configure):
 installed zfsutils-linux package post-installation script subprocess returned error exit status 1
dpkg: dependency problems prevent configuration of zfs-zed:
 zfs-zed depends on zfsutils-linux (>= 0.8.3-1ubuntu12); however:
  Package zfsutils-linux is not configured yet.

dpkg: error processing package zfs-zed (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 zfsutils-linux
 zfs-zed
E: Sub-process /usr/bin/dpkg returned an error code (1)

Additional information:

$ lsb_release -rd
Description: Ubuntu 20.04 LTS
Release: 20.04

$ apt-cache policy zfsutils-linux
zfsutils-linux:
  Installed: 0.8.3-1ubuntu12
  Candidate: 0.8.3-1ubuntu12
  Version table:
 *** 0.8.3-1ubuntu12 500
        500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
        100 /var/lib/dpkg/status