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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