[PATCH] Update Acorn partition parsing

[PATCH] Update Acorn partition parsing

This patch:
- re-enables cumana partition parsing
- adds eesox partition parsing
- makes the powertec partition parsing fail if sector 0 looks like
  a PC bios partition table

Rather than having a single "acorn_partition" parser for all these
types, we list them explicitly in check.c instead, along with some
explaination about why they're where they are.