Correctly acknowledge receipt of dbus messages. This prevents a hang on the sending end.

The dbus handler inadvertently returns DBUS_HANDLER_RESULT_NOT_YET_HANDLED for all dbus methods, whether actually handled or not. As a result, in a scanbd/scanbm setup using dbus for signalling, often scanning will delay for long periods or fail as scanbm doesn't get a timely confirmation that it has acquired or released the scanner from scanbd properly.

This commit acknowledges handled methods with an empty message, fixing the issue.