MFC r350058 (by imp): Implement {io,admin}-passthru commands.

These are mostly compatible with Linux, with three exceptions.

1. We don't do metadata segment stuff. Our passthrough interface doesn't cope. The code is there, but generates an error.
2. Linux lets you specify a namespace ID for the command. We current do not: we get ours from the namespace device, or pass in a generic one. Generally, this will lead to the same command, but FreeBSD's is safer since you can't specify the wrong id.
3. --show-command outputs to stderr instead of stdout so you can both see your command, and capture its output with a simple redirect.