Largish re-work of how dependencies are handled. The "start" and

Largish re-work of how dependencies are handled. The "start" and
"dependentStopped" methods were overloaded in unsound ways.

Add require() and release() methods to make an explicit requirement
on a dependency. A require() is added when the service is explictly
started, or when it is a dependency of such a service (directly),
or when it is a dependency of a service which has been required.