Fix undo for schemes that have internal partitions. Internal partitions do not constitute user-visible or active partitions and as such should not prevent undoing pending operations.
Fix undo for schemes that have internal partitions. Internal partitions
do not constitute user-visible or active partitions and as such should
not prevent undoing pending operations.

While here, initialize the last usable sector for the placeholder geom
based on the null scheme, created to allow undoing the destruction of
a scheme. This gives consistent output with "gpart show".

Based on a patch from:	"Andrey V. Elsukov" <bu7cher@yandex.ru>

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase