core: rework device_found_node() prototype

core: rework device_found_node() prototype

let's drop the "now" argument, it's exactly what MANAGER_IS_RUNNING()
returns, hence let's use that instead to simplify things.

Moreover, let's change the add/found argument pair to become found/mask,
which allows us to change multiple flags at the same time into opposing
directions, which will be useful later on.

Also, let's change the return type to void. It's a notifier call where
callers will ignore the return value anyway as it is nothing actionable.

Should not change behaviour.