Missing control commands

This is a list of control commands that I think should be supported in the future. Some of these are straight forward, and only miss code in the control interface parts. Others need more code / changes to the code that manipulates the unit set.

## Start/stop differences
Starting and stopping of units currently behaves differently with respect to how unsatisfied dependencies are handled.
1. Starting units: Currently the user has to start units one by one themselves
1. Stopping units: Currently stopping a unit will always stop all units that depend on it.

These should have two extra commands which should make the semantics clearer:
1. stop
1. stop-all
1. start
1. start-all

## Start newly loaded units
After reloading units, there should be a way to start all units, that have never been started.
1. start-all-new

## General queries about possible consequences of actions
Additionally there should be more queries available

1. To start unit A, which other units need to be started
1. To stop unit A, which other units need to be stopped
1. To remove unit A, which other units need to be removed
1. To remove unit A, which other units need to be stopped

Start with the control interface and the code that manipulates the unit set, which the issue identifies as the affected areas. Clarify the semantics for stop/start, newly loaded units, and consequence queries before implementation. Done means the listed commands and queries are supported with consistent dependency behavior.

Written by the indexing model from the issue text.