udevadm: make use of the new uuid-enabled triggering for "udevadm trigger"

This adds two things:

- A new switch --uuid is added to "udevadm trigger". If specified a
  random UUID is associated with the synthettic uevent and it is printed
  to stdout. It may then be used manually to match up uevents as they
  propagate through the system.

- The UUID logic is now implicitly enabled if "udevadm trigger --settle"
  is used, in order to wait for precisely the uevents we actually
  trigger. Fallback support is kept for pre-4.13 kernels (where the
  requests for trigger uevents with uuids results in EINVAL).