bsdinstall: Remove support for ZFS + MBR disk layouts

[emaste]

Summary

It hasn't worked for some time -- as reported in review D40816
"Installing FreeBSD with Auto ZFS + MBR has been broken ever since the
move to OpenZFS with FreeBSD 13."  It relied on the partition table and
ZFS data overlapping in a very fragile way and is not a good idea.

[emaste]

usr.sbin/bsdinstall/scripts/zfsboot

1674

[jhb]

If you want to remove it outright, that's probably a bigger change? I would say we should probably remove it entirely from bsdinstall, then probably stop building the non-GPT zfsboot as well.

[emaste]

Remove MBR bits

[jhb]

We will probably want a general RELNOTES entry at some point about removing ZFS MBR boot support?

[jhb]

A few more stragglers. I would maybe reword the commit log as "bsdinstall: Remove support for ZFS + MBR disk layouts".

Note that this means you can't use ZFS + MBR on a non-booting disk, but that seems ok to me. There's no good reason to do that anyway. You can always use the raw disk or GPT on non-boot disks.

usr.sbin/bsdinstall/scripts/zfsboot

828

[jhb]

Hmm, my suggestion to remove the invocation of sysctl kern.geom.part.mbr.enforce_chs=0 wasn't posted, but that should be removed as well.

[emaste]

rebase, incorporate feedback from @jhb

[jhb]

Let's land this. I can work on removing /boot/zfsboot afterwards.