Make GPT_read() obtain and validate the on-disk MBR itself, via

Make GPT_read() obtain and validate the on-disk MBR itself, via
MBR_read(), and report success/failure.

Simplifies logic and makes clearer that the protective MBR is a
required part of a GPT.

With the standardization on 0/-1 return values for helper
functions, rename 'valid' to 'error' in GPT_read() to make logic
less mind bending.

No functional change.