Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Allow bootstrapping mkimg on macOS/Linux

On these systems the (u)int64_t typedefs will not be implicitly defined by the
previous includes, so include <stdint.h> in the header that uses uint64_t.

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D23202

Make image_copyout_zeroes() an interface function.

Add image_data() for checking whether a sequence of blocks has data.
Use this for VHD and VMDK to avoid allocating space in the image
for empty sectors.

Note that this negatively affects performance because mkimg uses a
temporary file for the intermediate storage. When mkimg has better
internal book keeping, performance can be significantly improved.

Add VHD support to mkimg(1). VHD is used by Xen and Microsoft's Hyper-V
among others.

Add an undocumented option for unit testing (-y). When given, the image
will have UUIDs and timestamps synthesized in a way that gives identical
results across runs. As such, UUIDs stop being unique, globally or
otherwise.

VHD support requested by: gjb@

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.