This was needed for recover implementation.
Implement the recover command for GPT. Now GPT will marked as
corrupt when any of three types of corruption will be detected:
1. Damaged primary GPT header or table
2. Damaged secondary GPT header or table
3. Secondary header is not located in the last LBA
Marked GPT becomes read-only. Any changes with corrupt table
are prohibited. Only "destroy" and "recover" commands are allowed.
Discussed with: geom@ (mostly silence)
Tested by: Ilya A. Arhipov
Approved by: mav (mentor)
MFC after: 2 weeks