Adds an additive `register_driver_shared(Arc<dyn Driver>)` method
to `DeviceManager` so the caller (driver-manager) can pass a
pre-shared `Arc<DriverConfig>` and retain a `Weak` reference
to the SAME allocation. The SIGCHLD reaper's `Weak::upgrade()`
then resolves to the same maps that `probe()` populates, fixing
the long-standing Q1 dead-driver-leak where the reaper operated
on stale clones with empty maps.
vasilito
769bc4a9af