Recognize tagged metadata items and use for UNIX data
This is undocumented, but the Microsoft implementation seems to allow
variable-length, tagged metadata items to be appended to WIM dentries.
Currently it uses them for Object IDs, which DISM (Windows 8.1) will
backup and restore by default.
This commit adds support for these items, so they can be read and written
unmodified.
In addition, for our UNIX data extension, instead of storing the UNIX
data in the reserved fields of the dentry or in alternate data streams,
store it in a custom tagged item with a randomly chosen tag. This is
perhaps the best choice compatibility-wise.