Instead of copulating with newfs to produce a new FFS image into
memory, mmap() a given existing image and pass that as the backing
store. If -s is given, mmap is done with MAP_SHARED and changes
are kept across mounts, else MAP_COPY (i.e. MAP_PRIVATE for us) is
done and changes are lost when the server exits.
Note: -s does not guarantee any kind of file system safety whatsoever
and in case of kill, crash, exit or other form of elusion,
everything will be, according to our theme, quite screwed.
