Add 'mbr_' prefix to struct mbr field names.

Add 'mbr_' prefix to struct mbr field names.

Change 'offset' to mbr_lba_self and 'reloffset'
to mbr_lba_firstembr to make their use more evident.
Adjust a few parameter names to match.

Change 'part[]' to mbr_prt[] to reflect that it is an
array of struct prt.

No functional change.