MAXPARTITIONS increased from 8 to 16 for VAX

[David Brownlee <abs%absd.org@localhost>]

For all you staring at that RD52 and wondering how to make full use of
its capacity with only seven partitions, your solution is here!
No more will you have to nest devices to allow you to create your 15
separate 2MB partitions, now NetBSD/vax has been extended from 8
partitions per device to 16! (*)

(*) Also works for devices greater than 31MB!

After updating current all existing /dev nodes should be unaffected,
just run MAKEDEV to gain access to the additional partitions.

Commit message:
- Increase MAXPARTITIONS for vax from 8 to 16, using the standard NetBSD
  mechanism to ensure all existing /dev nodes continue to work
- Adjust boot block layout to fit additional partitions
- Adjust number of inodes on install media