iface-modem: allow cancellability during unlock required checks

The process doing the unlock required checks may need a lot of retries e.g. to decide whether a SIM card is available or not. If we do a quick SIM eject, so the unlock required check starts looping, and then insert the SIM again, we expect the loop to be cancelled right away, so that the new modem object can be reprobed without any interference from the old modem object.

We now take the modem-wide cancellable and bind it to the GTask in mm_iface_modem_update_lock_info(), and we pass it down to every sub-step of the async logic in the operation. We also plug the cancellable to the delayed retries in the interface logic, to allow aborting the checks right away.