device: do not merge devices
Don't try to merge devices that have been created via dependencies when they appear in the system and can be recognized as the same. Instead, simply continue to maintain them independently of each other, however with the same state cycle. Why? Because otherwise we'd have a hard time to seperate the dependencies after the devices are unplugged again and we hence cannot be sure anymore that next time the device is plugged in it will carry the same names. Example: if one depndency refers to dev-sda.device and another one to dev-by-id-xxxyyy.device we only learn at time of plug in of the device that it is actually the same device that was ment. In the moment the device is unplugged again we won't know anymore their relation to each other and the next time the harddisk is plugged it might even appear as dev-by-id-xxxyyy.device and dev-sdb.service. To ensure the dependencies continue to have the meaning they were intended to have let's hence keep the .device objects seperate all the time, even when they are plugged in. This patch also introduces a new Following= property which points from the various .device units of a specific device to the main .device unit for it. This can be used by the client side to figure out the relation of the .device units to each other and even filter units from display.
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