MFC r214352 adapted to stable/8:

MFC r214352 adapted to stable/8:
  Reimplemented "gpart destroy -F". Now it does all work in kernel.
  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.

Approved by:	kib (mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/8@214975 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f