rc: Add a zpoolreguid rc.d script

[markj]

Summary

If one boots up multiple copies of a template VM image containing a
zpool, the pool GUIDs will be identical, making it impossible to, e.g.,
share datasets between them.

This diff introduces a simple workaround for the problem: one can use
the script to, upon first boot, assign a new GUID to one or more zpools.

I have some WIP to create ZFS-based VM images from release(7). This
script would be enabled in such images to ensure that each VM instance
gets its own pool GUID.

[asomers]

This is going to be very useful. Will you create a matching change that will run zpool online -e on firstboot, or just assume that the VM images will be built with autoexpand=true?

[markj]

In D35336#801061, @asomers wrote:

Will you create a matching change that will run zpool online -e on firstboot, or just assume that the VM images will be built with autoexpand=true?

rc.d/growfs already does that. :)

[markj]

One other thing that might be useful is a script to enable some or all feature flags on a pool. The WIP makefs -t zfs implementation leaves all feature flags off.

[asomers]

In D35336#801073, @markj wrote:

One other thing that might be useful is a script to enable some or all feature flags on a pool. The WIP makefs -t zfs implementation leaves all feature flags off.

I don't think that needs to happen on firstboot. You know at build time what kernel will be running in the VM, so you can enable pool features there. Unless you want to conditionally enable certain features depending on how big the disk is, or what hypervisor is involved. But I don't think that's likely.

[pauamma_gundo.com]

Manual page English LGTM. Can't attest to correctness.

[allanjude]

Reviewed By: allanjude