usr.sbin/makefs: Add HAMMER2 offline read / VOP_READ support

usr.sbin/makefs: Add HAMMER2 offline read / VOP_READ support

The offline read takes HAMMER2 specific "-o R" option.
`image-file` argument is a valid HAMMER2 image file or block device.
Unlike other offline commands, `directory` must be a valid directory.

e.g.
$ makefs -t hammer2 -o R=/path/to/file /dev/adx /a/b/c
$ makefs -t hammer2 -o R=/path/to/file /path/to/hammer2.img /a/b/c

The "-o R" option requires file path argument. The file is retrieved
from `image-file` and written to a file under `directory`.
In the examples above, /path/to/file is retrieved as /a/b/c/file
on success.