udev: create /run/udev/queue file earlier (#40518)

When udevd received a uevent or inotify event, then we will queue
events. To minimize the race between processing these events by udevd
and user invocation of 'udevadm settle', let's create /run/udev/queue
file earlier.
On some errors, no event may be queued, but in that case, the file will
be removed by the post event source if nothing queued. See on_post().

Hopefully mitigate the issue #40499.