Add -n/--dry-run option

[wyoung]

While debugging the issues related to #219, I'd have liked the option to have it just print on screen what it planned to do, so I wouldn't have to go into the sub-filesystems of those I don't want being treated recursively and destroy the unwanted snapshots.

Standard practice is to implement this as -n and (optionally) as --dry-run.

[sparky3387]

FYI the --verbose, --readonly and --cron options combined create this

[Follpvosten]

I'd very much like such an option for syncoid, especially to test if my regexes for --exclude have the intended effect.

[dlangille]

EDIT: not a Duplicate of #11

[phreaker0]

@dlangille this is about sanoid, not syncoid. With the --readonly option this should already be solved @wyoung right?