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

[dieterbsd at engineer.com]

> Did you try to remove both deprecated GEOM_BSD and GEOM_MBR
> from your config?
> Now in 8.0+ you should use GEOM_PART_MBR, GEOM_PART_EBR and
> GEOM_PART_BSD instead. And they all are in DEFAULTS config by
> default.
>
> Your MBR partitions should be served with GEOM_PART_MBR scheme.
> An extended partitions should be served with GEOM_PART_EBR scheme.
> A BSD slices should be served with GEOM_PART_BSD scheme.

I started with the default GENERIC 8.0-release kernel and it is broken.
It does not provide /dev entries for MBR extended partitions.

Changing the GEOM_* options to be

#define GEOM_BSD 1
#define GEOM_LABEL 1
#define GEOM_PART_GPT 1
#define GEOM_MBR 1

(which turns out to be the same as they were in 7.1) makes everything 
work.