As far as I can tell, the systemd snippet hasn't depended on groups
being initialized since 5d02c02 in
2015, when a `setfacl` call was removed.

Previously, the activation script was responsible for ensuring that
/etc/machine-id exists. However, the only time it could not already
exist is during stage-2-init, not while switching configurations,
because one of the first things systemd does when starting up as PID 1
is to create this file. So I've moved the initialization to
stage-2-init.

Furthermore, since systemd will do the equivalent of
systemd-machine-id-setup if /etc/machine-id doesn't have valid contents,
we don't need to do that ourselves.

We _do_, however, want to ensure that the file at least exists, because
systemd also uses the non-existence of this file to guess that this is a
first-boot situation. In that case, systemd tries to create some
symlinks in /etc/systemd/system according to its presets, which it can't
do because we've already populated /etc according to the current NixOS
configuration.

This is not necessary for any other activation script snippets, so it's
okay to do it after stage-2-init runs the activation script. None of
them declare a dependency on the "systemd" snippet. Also, most of them
only create files or directories in ways that obviously don't need the
machine-id set.

The default value for journald's Storage option is "auto", which
determines whether to log to /var/log/journal based on whether that
directory already exists. So NixOS has been unconditionally creating
that directory in activation scripts.

However, we can get the same behavior by configuring journald.conf to
set Storage to "persistent" instead. In that case, journald will create
the directory itself if necessary.

As far as I can tell, systemd has never used this directory, so I think
this is a holdover from before udev merged into systemd.

These don't need to get cleaned up during activation; that can wait
until systemd-tmpfiles-setup runs.

Nix 2.0 no longer uses these directories.

/run/nix/current-load was moved to /nix/var/nix/current-load in 2017
(Nix commit d7653df). Anyway,
src/build-remote/build-remote.cc will create the current-load directory
if it doesn't exist already.

/run/nix/remote-stores seems to have been deprecated since 2014 (Nix
commit b1af336) when the documentation
for $NIX_OTHER_STORES was removed, and support for it was dropped
entirely in 2016 (Nix commit 4494000).

Anything that uses OpenGL starts after sysinit.target, so
systemd-tmpfiles runs before anything that needs these symlinks.

I think pam_lastlog is the only thing that writes to these files in
practice on a modern Linux system, so in a configuration that doesn't
use that module, we don't need to create these files.

I used tmpfiles.d instead of activation snippets to create the logs.
It's good enough for upstream and other distros; it's probably good
enough for us.

Replace several activation script snippets with declarative configuration