Snapshot support

[jcnelson]

Add an ioctl() that forces an fskit-enabled app to atomically dump the state of its filesystem to disk.

This should be a matter of:

• write-lock the root inode
• explore the filesystem tree and dump inode state
• optionally call the app-defined read() callback (*) when exploring each regular file inode
• unlock the root inode

Step 3 should be optional at the caller's discretion, since despite whatever documentation and warnings we may write to advise against it, we can't feasibly guarantee that read() will be side-effect-free for a particular application.