Mimic systemd-udev-trigger.service when triggering udev events.

The default setting for `udevadm trigger' is --type=devices.
That's why systemd-udev-trigger.service runs it twice, once
with --type=subsystems, and then with --type=devices.

Given the racy behaviour I'm seeing in tails-unblock-network.service,
it seems worth doing exactly the same thing as systemd-udev-trigger does.