Allow appending to default props for zfs and zpool list.

[Shreshth3]

This PR allows users to add specific properties in addition to the defaults when using zfs list -o or zpool list -o. For example, zpool list -o +guid will run zpool list showing the default properties in addition to guid.

Resolves #17112.

One note: with the current implementation, if a user tries to append a property that is already in the defaults (e.g. name), they will see that column two times. If this is not acceptable, I can adjust the implementation accordingly.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[intelfx]

One note: with the current implementation, if a user tries to append a property that is already in the defaults (e.g. name), they will see that column two times. If this is not acceptable, I can adjust the implementation accordingly.

It would be nice to support this case by removing such a column from the "default" part of the list and inserting it into the "user" part of the list at the specified position. E.g. I might want to keep the mountpoint column last, and so I'd write zfs list -o +lused,lrefer,mountpoint.

Additionally, perhaps some care is needed to support repeated -o +... options; this is a non-intuitive restriction and might create problems when scripting around it.