Consider making non-ISO USB images using 'zpool checkpoint'

Addressing https://github.com/helloSystem/ISO/issues/4, in BSD Now Episode 383: "Scale the tail" it is explained that sing just ZFS it is **not** possible to generate a writeable pool from read-only media plus read-write media (such as RAM).

https://www.freebsdnews.com/2021/01/01/bsd-now-episode-383-scale-the-tail/

But it is suggested that since few people are using actual DVD media anymore, we could construct non-ISO disk images with ZFS on them (similar to what NomadBSD is doing), but then using **zpool checkpoint** (think of it as a "pool-wide snapshot").

During the boot process, one could go back to the last checkpoint.

Advantages:
* ~100 seconds boot time saved
* RAM requirement goes from 4GB to ~1GB

Disadvantages:
* Does not work from truly read-only media like DVDs
* Truly read-only media are preferred for auditing purposes ("I know for sure that no one tampered with the software")
* May not work for systems like [Ventoy](https://www.ventoy.net/en/distrowatch.html)

Maybe we can at least find a way to boot into Xorg from truly read-only media by mounting a few tmpfs in the right places like `/etc`, `/var`, `/usr/local/etc`, `/usr/local/var`? This way one could at least _install_ the system from a DVD. Is it worth the extra hassle? I don't know.