static-nodes: move creation of static nodes from udevd to tmpfiles

As of kmod v14, it is possible to export the static node information from
/lib/modules/`uname -r`/modules.devname in tmpfiles.d(5) format.

Use this functionality to let systemd-tmpfilesd create the static device nodes
at boot, and drop the functionality from systemd-udevd.

As an effect of this we can move from systemd-udevd to systemd-tmpfiles-setup-dev:

 * the conditional CAP_MKNOD (replaced by checking if /sys is mounted rw)
 * ordering before local-fs-pre.target (see 89d09e1b5c65a2d97840f682e0932c8bb499f166)