Implement filesystem-only checkpointing.

- `runsc fscheckpoint` saves a filesystem checkpoint, analogous to `runsc
    checkpoint`. `-direct` and `-leave-running` flags are supported for
    compatibility with `runsc checkpoint`. `-save-restore-exec*` is currently
    not supported due to lack of a use case. `-compression` support is not
    planned (specific files in filesystem checkpoint images may use compression
    as appropriate, but this is an implementation detail).
    `-exclude-committed-zero-pages` is inapplicable.

  - `runsc create -fs-restore-image-path` restores a filesystem checkpoint. The
    `-fs-restore-direct` flag has the same effect as `runsc restore -direct`.
    The behavior of `runsc restore -background` is always in effect for
    filesystem checkpoints.

PiperOrigin-RevId: 885914969