kern/141235: 8.0 no longer provides /dev entries for all disk slices [regression]

[dieterbsd at engineer.com]

> You should remove GEOM_PART_* entries. I think that the duplicates you
> saw are because you had geom_part still enabled.

I thought about that.  Having both GEOM_PART_MBR and GEOM_MBR
seemed suspicious, but GEOM_PART_MBR isn't from the config file:

grep -i geom GENERIC
options         GEOM_PART_GPT           # GUID Partition Tables.
options         GEOM_LABEL              # Provides labelization

and then I added GEOM_MBR and GEOM_BSD as suggested.  So I don't
know where GEOM_PART_MBR comes from or how to get rid of it.
I am assuming that "make buildkernel" calls config(8) and config
builds opt_geom.h based on the config file, but it must have some
other input that I haven't found.

It isn't clear to me what the difference between GEOM_PART_MBR and
GEOM_MBR is supposed to be (same for GEOM_PART_BSD and GEOM_BSD).
conf/NOTES doesn't list a GEOM_GPT, only GEOM_PART_GPT.  I assume
that removing GEOM_PART_GPT would break disks using GUID partition
tables.