Reloading of units

Rustysd needs a control interface command that triggers a complete reload of all units, which integrates changed / new units into the running system.

Adding and activating new units should be done similarly to the initial startup by walking the dependency graph, but ignoring unchanged units. Changed units should be killed and restarted with the new configuration

Currently there is only support for adding new units one by one which is fine for manual enabling of services but not for automatically adding a bunch of services with dependencies between them.

Start with the existing control-interface command for adding units and the initial-startup dependency-graph traversal. Trace how units are detected, compared, stopped, restarted, and activated, then define completion as a reload command that incorporates new and changed units while leaving unchanged units running.

Written by the indexing model from the issue text.