udev: export tags of "dead" device nodes to /run/udev/static_node-tags/

Based on a patch by Kay Sievers.

A tag is exported at boot as a symlinks to the device node in the folder
/run/udev/static_node-tags/<tagname>/, if the device node exists.

These tags are cleaned up by udevadm info --cleanup-db, but are otherwise
never removed.
logind: apply ACL's to "dead" device nodes

Based on a patch by Kay Sievers.

When a dead device nodes is tagged with "uaccess" using the static_node mechanism,
it's ACL's are managed by logind in the same way as "live" device nodes.

This allows in particular /dev/snd/{seq,timer} to cause modules to be loaded
on-demand when accessed by a non-privileged user.