* Required because some users appear to force kill Rufus while we're doing WUE patching of boot.wim,
and Windows prevents a .wim with the same path and index from being mounted twice, even if the
original .wim has become stale or deleted. Oh, and of course the WIM APIs don't have a force-mount
flag that would take care of this whole situation.
* Basically, this forces us to parse HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WIMMount\Mounted Images
and check each instance for a .wim/index match, so that we can access to the existing mount path
so that we can actually unmout the image (because, in typical Microsoft fashion, WIMUnmountImage
requires both the mount path and the source image to be provided).
* Closes#2199.
* Also improve the existing VHD code to use a struct where possible.
* Some "unofficial" Windows ISOs use a custom boot.wim that only includes the Setup
image at index 1, rather than at index 2, after the PE image, for official ISOs.
* Also refactor to add a long needed vhd.h header.
* Also fix a MinGW warning.
