tmpfiles: create static device nodes before udev is started

Since v183, the contents of /usr/lib/udev/devices is no longer copied to /dev
on boot, rather systemd-tmpfiles should be used instead. However, as
systemd-tmpfiles --create is only ran long after udevd has been started, it is
no longer possible to use udev rules to assign permissions to the static nodes.

This calls systemd-tmpfiles --create early, before udev is started, and
restricts the call to /dev, which is known to be mounted already.

In the future, this could also take over the creation of static device nodes
from systemd-udevd.