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

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

Add a partition type for nandfs to the apm, bsd, gpt and vtoc8 schemes.
The gpart alias for these partition types is "freebsd-nandfs".

- In gpart_bootfile_read() fix an off-by-one error preventing the bootstrap
file to be of maximum size.
- Add special handling required for SMI/VTOC8 disklabel partcode, i.e. avoid
overwriting the label when writing the bootstrap code to the partition
starting at 0 and install it to all partitions when the -i option is omitted
just like geom_sunlabel(4) and sunlabel(8) do by default.
- Add missing prototypes.
- Add const where applicable.

Reviewed by: marcel
MFC after: 3 days

Mark the vtoc.h structure as packed so that it is the right size and
layout on arm.

MFC after: 7 days

Add __packed to the part sub-structure to compensate for
the 32-bit default structure alignment on ARM.

Add support for VTOC8 labels (aka sun disk labels). When a label does
not have VTOC information about the partitions, it will be created.
This is because the VTOC information is used for the partition type
and FreeBSD's sunlabel(8) does not create nor use VTOC information.
For this purpose, new tags have been added to support FreeBSD's
partition types.