Here's a suggestion to avoid these problems: Change disklabel's idea
of the `source of truth' to be the MBR, not the operator.

Before Perry's SSTO bootfloppy (applause) there wasn't space for much
in the way of install tools. In that world it made sense for the i386
disklabel to do whatever the operator said.

That leaves a lot of space to make mistakes: IMHO, it leaves too much
space for multi-boot installations.

Now we have more flexibliity in the install tools, it should be
possible to change disklabel so that it implicitly beleives whatever
is in the MBR, and doesn't default to overriding that with wih values
from the given disklabel.

The installation instructions would say that dedicated installation
should use fdisk to zap out all MBR partitions, and then reboot
and restart.

If we need to leave space for OS-BS or installboot, the fake-disklabel
code should allocate that space in its faked-up disklabel (for empty
MBRs only).  That way most users never need to know about it at all,
and those who really want to handcraft things can do it with fdisk.

Comments?