udev: sort queued events by their seqnum

udev: sort queued events by their seqnum

Unfortunately, the kernel may send events in a random order:
```
[   25.769624] systemd-udevd[194]: sdi7: Device is queued (SEQNUM=2843, ACTION=add)
[   25.769893] systemd-udevd[194]: sda5: Device is queued (SEQNUM=2842, ACTION=add)
[   25.770517] systemd-udevd[194]: sdi8: Device is queued (SEQNUM=2844, ACTION=add)
```
As you can see, udevd receives the event with SEQNUM=2843 earlier than
one with SEQNUM=2842.

Let's make queued events sorted, as our logic of determining which event
is ready for being processed assumes that queued events are sorted.
See event_build_dependencies().

Also, refuse to queue an event if another event with the same seqnum is
already queued.