Tail space reclamation issue

Due to a variety of issues (Subiquity's lack of support for virtual block devices as installation target, Ubiquity's shenanigans with ZFS devices), the installer creates a temporary partition to install the O/S, then copies it to the filesystem in the root pool.

If the user choose not to leave any tail space (or to leave less than that required for the temporary O/S installation), the ZFS installer attempted to reclaim the space).

While the reclamation works on paper (ZFS should be able to extend without problems), in practice, there is a minority of setups where this didn't work. Unfortunately, such cases typically (=always) can't be reproduced locally, and are very hard (=practically impossible) to debug.

See issues #157 and #104.

Therefore, it's been chosen to:

• not do anything if the user chooses a tail space greater than the required temporary O/S installation;
• leave the tail space as is if it's less than required.

Users can still claim the space after the installation; the procedure is actually simple (run from a live media):

zpool export rpool

# If on multiple disks, repeat for each disk.
#
parted -s /path/to/disk_dev rm 4
parted -s /path/to/disk_dev unit s resizepart 3 -- 100%

zpool import -l rpool