Add client support (-d, --dry-run) for performing noop queries on a
sMAP server.

N.B. -- older sMAP servers will ignore this flag and we don't
currently have a versioning scheme for the sMAP API that can be
enforced on requests, and so this could cause you to mutate tags on
older servers when you didn't mean to.
Move all docs to en/2.0 to prep for documenting the change to the set
commands, other publically visible changes.