MFC r263468:
  When loader(8) inspects MBR, it chooses GPT as main partition table,
  when MBR contains only PMBR entry or it is bootcamp-compatible.
  If MBR has PMBR entry and some other, the loader rejects it.

  Make these checks to be less strict. If loader decided that PMBR
  isn't suitable for GPT, it will use MBR.

  Reported by:	Paul Thornton