Automatic fallback to in-process mode when D-Bus not available

[romangg]

Currently one has to set the environment variable DISMAN_IN_PROCESS to have the consumer run the backend in-process. Even when D-Bus is not available and trying to have an out-of-process backend would fail.

We could do an automatic fallback in this case to an in-process backend.

On the other side Disman without the D-Bus backend does not always work correctly with more than one consumer and this might be a hidden issue then. Requiring to set the environment variable manually makes sure the user knows what he is getting himself into.

Also in most cases D-Bus should be available. Are there counter-examples?