sd-device: introduce sd_device_open()

sd-device: introduce sd_device_open()

We usually open() device node obtained by sd_device_get_devname().
However, the device node corresponds to the sd-device object may be
already removed, and another device node with the same path may be
created, hence an unexpected device may be opened.

The sd_device_open() opens device node, and checks the devnum and
diskseq of opened devnum, to avoid the above possibility.

Prompted by https://github.com/systemd/systemd/issues/22906#issuecomment-1082736443.