usr.sbin/makefs: Add HAMMER2 offline growfs support

usr.sbin/makefs: Add HAMMER2 offline growfs support

The offline growfs takes HAMMER2 specific "-o G" 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 G /dev/adx --
$ makefs -t hammer2 -o G /path/to/hammer2.img --

By default expands volume to image file or block device size.
If -s option is specified, use that size.
No shrink support in the same way hammer2(8) growfs doesn't.