Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r270445:
The size of the GPT table can not be less than one sector.

MFC r270521:
Since the size of GPT entry may differ from the sizeof(struct gpt_ent),
use the size from GPT header to iterate entries.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make the check for number of entries less strict.
Some partitioning tools can create GPT with number of entries less
than 128.

MFC after: 1 week

Add bootcamp support to the loader.

Tested by: dchagin
MFC after: 1 week

The MBR data is not necessarily aligned. This is a problem on ARM.

Add comment why the code has been disabled.

Requested by: rpaulo

Some BIOSes return incorrect number of sectors, make checks less
strictly, to do not lost some partitions.

Reported by: swills@

As it turned out, there are some installations, where BSD label
contains partitions with type zero. And it has worked.
So, allow detect these partitions.

Reported by: glebius

Fix start offset calculation for the EBR partitions.

Create the interface to work with various partition tables from the
loader(8). The following partition tables are supported: BSD label, GPT,
MBR, EBR and VTOC8.