udev: readjust priorities of event sources

udev: readjust priorities of event sources

Follow-up for 511619087b66baa52907d1f6c25e28ccb9590a5f.

Notable changes are
- SIGTERM is the highest among others, to make not udevd queue too
  many events, as we need to serialize them anyway.
- device monitor has the second highest priority, to make 'remove'
  uevents received earlier than IN_IGNORED inotify events. Otherwise,
  after IN_IGNORED is received, if there is no queued event,
  /run/udev/queue file will be removed by the post-event source of the
  inotify event, and 'udevadm settle' or friends may wrongly finish,
  even we will soon queue 'remove' uevents for the device.
  This change should fix the recent instability of TEST-64-UDEV-STORAGE.

For other changes, see the comments in the code.