Remove the dependency on the kernel -- in particular the gctl request to
the GEOM_BSD class -- to translate the absolute offsets in the label to
relative ones. This makes bslabel(8) work correctly with GEOM_PART and
also when the BSD label is nested under arbitrary partitioning schemes.
Inspired by: Eygene Ryabinkin <rea-fbsd@codelabs.ru>
Approved by: re (kib)
9fdef7744d