graphman: Improve `unused list`

[lutter]

Currently, graphman unused list will list all unused deployments, including ones that were previously removed, and unused list -e shows the ones that are queued for removal but still exist.

It would be better if unused list only shows what will be removed, and something else (maybe unused list -a) prints the auditing trail of removals ever done.

[kaiwetlesen]

Ideally it'd be more useful for the default behaviour for graphman unused list to correspond to the behaviour of graphman unused list -e, with the -e switch being preserved to accommodate any scripts. Then, to enable the current default behaviour of printing all schemata, an --all / -a flag would be appropriate and match with common CLI conventions.