VFS plugin to replicate from one file-system to another. Can be used to
seed a mutable file-system with an immutable file-system. The plugin is
configure with two paths, a read-only path, and a read-write path. This
is an initial implementation that copies files on open. It is not
optimized to perform actual copy-on-write, but the result is the same.
<vfs>
<dir name="immutable"> ... </dir>
<dir name="mutable"> ... </dir>
<dir name="cow">
<cow ro="/immutable" rw="/mutable"/>
</dir>
</vfs>
Fix#2745
