libvtreefs: convert to KNF

Change-Id: I81bdf05c9b630a0cbb0ac573d36d4f59f8137199
libvtreefs: API changes/extensions, part 1

- move primary I/O buffer into vtreefs; change read hook API;
- add hooks for write, truncate, symlink, mknod, unlink, chmod/chown;
- modernize message_hook;
- change procfs, devman, gpio accordingly;

Change-Id: I9f0669e41195efa3253032e95d93f0a78e9d68d6
libvtreefs: API changes/extensions, part 2

- rename start_vtreefs to run_vtreefs, since the function returns upon
  termination these days;
- add get_inode_slots function to retrieve the number of indexed slots;
- add support for extra per-inode data for arbitrary storage.

Change-Id: If2d365d7b478a1cecc9e20fb2b3e70c1a1cf7243