usr.sbin/makefs: Add HAMMER2 offline inode get support
The offline inode takes HAMMER2 specific "-o I" option.
`image-file` argument is a valid HAMMER2 image file or block device.
`directory` argument is unused, but needs to be a valid path or "--".
e.g.
$ makefs -t hammer2 -o I=get:/path/to/file /dev/adx --
$ makefs -t hammer2 -o I=get:/path/to/file /path/to/hammer2.img --
The "-o I" option requires inode command name argument, which is
currently only "get". "get" takes `:<inode_path>` string after command
name. "get" is equivalent to hammer2(8) printinode directive.