Properly handle device aliases used as dependencies

Properly handle device aliases used as dependencies

If a device unit has aliases defined in udev rules, and there are
other units that depend on that alias, as in

  BindTo=sys-subsystem-net-devices-eth0.device

then systemd will fail the start the alias, and any dependent units
will time out.  See

  https://bugs.freedesktop.org/show_bug.cgi?id=52580

This is because unit_add_name() in device_add_escaped_name() will
return EEXIST.

The solution taken here is to call device_update_unit() on the alias
name.  Thus if a unit with the alias name already exists, we reuse it;
otherwise a new unit is created.  Creating multiple units for a single
device is perhaps suboptimal, but it's consistent with the treatment
of udev symlinks in device_process_new_device().