(Do we offset partition a to avoid clobbering the boostrap code?)

Yes.  The hp300 boot blocks are too large to fit in the standard 8k
offset before the superblock.  On the i386 port, you don't need to
offset the `c' partition.

   i386:
   |------------d-----------------------------|
   P  |---------c----------------||-other-os--|
      |--a---||--b---||----e-----|

The `c' partition should correspond exactly to the NetBSD partition in
the DOS partition table, which may be the whole disk.