turn rc.local into a systemd service

We have a somewhat strange rc.local configuration file which does this:

##
## THIS FILE IS UNDER PUPPET CONTROL. DON'T EDIT IT HERE.
## USE: git clone git+ssh://$USER@puppet.debian.org/srv/puppet.debian.org/git/dsa-puppet.git
##

if [ -e /proc/sys/kernel/modules_disabled ]; then
	( sleep 60;
	  echo 1 > /proc/sys/kernel/modules_disabled || true
	) & disown
fi

touch /var/run/reboot-lock

Here's what it does, from what I can gather.

1. If the kernel is so configured (if /proc/sys/kernel/modules_disabled is present) it will disable module loading, after a 60 seconds delay
2. it creates the /var/run/reboot-lock file, which is used by other components to forbid reboots through a molly-guard hook

Those are two independent purposes, of course.

This, or at least the first part, should be replaced by a systemd service. This will make it easier to disable, which is necessary when we want to actually load modules. This is done in the buster upgrade process, for example, which says:

5. Enable module loading (for ferm)

sed -i -e 's/.*modules_disabled/#&/' /etc/rc.local

reboot

export LC_ALL=C.UTF-8 &&
sudo ttyrec -e screen /var/log/upgrade-buster.ttyrec.2

That sed line has a serious bug which will make rc.local crash during the reboot. Instead of just "disabling the disabling", it actually mangles the shell script and turns the if block into this:

#if [ -e /proc/sys/kernel/modules_disabled ]; then
        ( sleep 60;
#         echo 1 > /proc/sys/kernel/modules_disabled || true
        ) & disown
fi

And while that hack could be fixed, it would be much easier, logical and understandable if it was written as (say) systemctl disable module-disable. There might even be existing service files that do this which we could use.

The reboot lock, on its part, could be created by the systemd.tmpfiles mechanism. This, in turn, would make it effectively as soon as Puppet runs, as opposed to after the first reboot in the old mechanism.