udev: serialize queued events on exit (#37047)

udev: serialize queued events on exit (#37047)

Previously, all queued events were discarded on exit, hence several
events might not be processed by udevd when it is restarted. Such
situation especially easily happens on switching root.

This makes queued events serialized on exit, and deserialized in the
next invocation. Hence, no events should be lost during restarting
udevd.

This is important with the several aspects. Basically
systemd-udev-trigger.service (re)triggers all devices anyway after
switching root, But the service may be disabled or modified by admin.
Moreover, the service produces only 'add' events, and thus the service
cannot cover events with other actions generated by the kernel during
switching root. Also, the userspace triggered events may not contain
some parameters compared with events triggered by the kernel.