In preparation for adding inode check-hashes, clean up and
document the libufs interface for fetching and storing inodes.
The undocumented getino / putino interface has been replaced
with a new getinode / putinode interface.

Convert the utilities that had been using the undocumented
interface to use the new documented interface.

No functional change (as for now the libufs library does not
do inode check-hashes).

Reviewed by: kib
Tested by: Peter Holm
Sponsored by: Netflix