Shutdown hooks are executed right before the shutdown, which is useful
for some applications. Among other things this is needed for mdadm hook
to run.

We also correct paths in udev rules. This is cleaner and allows for less
unexpected behaviour. We still check for self-references, however we do
it manually now and only for binaries as udev rules can have them.
Rather, we patch them out during initrd generation now.

We don't patch basename and readlink now too as they were added for
mdadm in 8ecd3a5.

This fixes a serious bug on NixOS with swraid where mdadm arrays weren't
properly stopped on shutdown. Rather than fixing the unit by adding
`Before=final.target` we completely move to upstream units, which uses
systemd shutdown hooks instead. This also drives down maintenance costs
for us.

Use systemd.packages instead, it's less error prone and more in line with
what's expected.