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 269745: Create a redundant grain directory and table.

MFC r268236,268264,268524,268646,268802,269021:
This brings VHD support to mkimg(1); both dynamic and fixed file formats.
Dynamic VHD and VMDK file images are now sparsely written, meaning that
"free" sectors do not occupy space.

Relnotes: yes

MFC mkimg(1) -- revisions 268159, 268134, 266556, 266514, 266513,
266512, 266511, 266510, 266509, 266176, 265468, 265467,
265462, 265170, 263926, 263924, 263923, 263919 and 263918.

Revision 267182 changed mkimg.1 alongside other unrelated manpages.
The change to mkimg.1 has been applied without registering a merge
of the revision. This allows a future merge of r267182 to happen.

Relnotes: yes

MFuser/marcel/mkimg:
Add support for different output formats:
1. The output file that was previously written is now called the raw format.
2. Add the vmdk output format to create VMDK images.

When the format is not given, the raw output format is assumed.

Replace unchecked calls to write(2) and lseek(2) with calls to
sparse_write() and make sure to check for errors. In particular,
lseek(2) may not be possible (e.g. the output file is stdout)
and sparse_write(2) will do what is optimal (i.e. use lseek(2)
when possible).

Check allocation errors and free memory we allocated.

As it appears the image must be rounded to the nearest megabyte,

Commit rough but working code:
o The VMDK image can be queried and comverted by qemu-img
o VMware Fusion finds no errors when recovering using
'vmware-vdiskmanager -R' and the disk can be added to any
vortual machine.

Give output formats a chance to (re-)size the image before the
scheme adds the partitioning metadata. This is needed by VMDK
to round the image size to the grain size.

Add image_get_size() so formats can query about the image size.
Create the VMDK (embedded) descriptor file and round its size
to a multiple of 512.

TODO: We need the name of the output file -- it's in the extent
description. For now, just use "mkimg.vmdk".

Define the sparse extent header and document the steps to take
to create the VMDK file. Add some notes as well.

What I needed to know (roughly) is what interface the image API
is to provide and whether there are (knowledge) gaps that prevent
me from implementing it for real.

Add a linker set for output formats. This is mostly copied and trimmed
from the linker set for schemes. Add stubs for raw and vmdk output
formats so as to demonstrate that the usage message is correct.