MFC r292057:

MFC r292057:
  Make detection of GPT a bit more reliable.

  When we are detecting a partition table and didn't find PMBR, try to
  read backup GPT header from the last sector and if it is correct,
  assume that we have GPT.

  Differential Revision:	https://reviews.freebsd.org/D4282

MFC r292058:
  Remove a note about damaged PMBR. Now GPT will be detected automatically
  with such corruption.

Relnotes:	yes

git-svn-id: svn://svn.freebsd.org/base/stable/10@293634 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f