o BSD disklabels have relative offsets. Even for the BSD in MBR slice
setup, except when the mbroffset ioctl is supported. Since we don't
support that ioctl, bsdlabel(8) expects relative offsets. So, when
reading an existing disklabel, correct for disklabels that mistakenly
have the mbroffset offsets.
o Don't take the geometry seriously, because it's untrustworthy. We do
expect the numbers to be within range. This means that the secperunit
field will not be computed from secpercyl and ncyls, but simply is
the mediasize in sectors.
o Don't enforce partitions to be aligned to track boundaries. The
default label, constructed by bsdlabel(8), puts partition a at offset
BBSIZE bytes, which commonly means sector 16.
392ffade03