Partitions>8: Increase the number of supported partitions from 8 to 16.

Partitions>8: Increase the number of supported partitions from 8 to 16.
Decrease the number of supported slices from 32 to 16.  Note that the
5.x boot2 code, which we adopted, was being installed just after the old
disklabel.  This commit moves the boot code install to the next logical
sector (aka 4.x) in order to accomodate the larger label.

Fix newfs to not hardcode 'h' as the last partition.

Also modify 'disklabel' to not complain about preexisting garbage past
partition #8, and to detect and refuse to overwrite the old bootcode
with the new larger label until after you have installed new boot code.