vfs: add snapshot create/delete hooks

vfs: add snapshot create/delete hooks

This change adds three new VFS hooks covering snapshot manipulation:
- snap_check_path
  Check whether a path supports snapshots.
- snap_create
  Request the creation of a snapshot of the provided path.
- snap_delete
  Request the deletion of a snapshot.

These VFS call-outs will be used in future by Samba's File Server Remote
VSS Protocol (FSRVP) server.

MS-FSVRP states:
  At any given time, Windows servers allow only one shadow copy set to
  be going through the creation process.
Therefore, only provide synchronous hooks for now, which can be
converted to asynchronous _send/_recv functions when the corresponding
DCE/RPC server infrastructure is in place.

Signed-off-by: David Disseldorp <ddiss@samba.org>