usr.sbin/makefs: Add HAMMER2 offline PFS get|lookup|create|delete|snapshot support
The offline PFS takes HAMMER2 specific "-o P" 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 P=get /dev/adx --
$ makefs -t hammer2 -o P=lookup:name /path/to/hammer2.img --
$ makefs -t hammer2 -o P=create:name /dev/adx --
$ makefs -t hammer2 -o P=delete:name /path/to/hammer2.img --
$ makefs -t hammer2 -o P=snapshot:name /dev/adx --
$ makefs -t hammer2 -o P=snapshot /path/to/hammer2.img --
The "-o P" option requires PFS command name argument, which are
"get" (alias "list"), "lookup", "create", "delete" and "snapshot".
lookup, create and delete take `:<pfs_name>` string after command name.
snapshot takes optional `:<pfs_name>` string after command name.
Use -m option (default "DATA") to specify target PFS for snapshot.