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 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.

Have the format resize twice. The first time is before we call
scheme_write(). The second time is immediately before we call
the format's write function. The first call is needed to have
the scheme know the right image size. The second call is needed
to compensate for the scheme adjusting the size while writing.

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 format_write() that calls the write() function of the output
format. Have the raw format use image_copyout() for now.

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.