nixos/switch-to-configuration: reload user units

When rebuilding you have to manually run `systemctl --user
daemon-reload`. It gathers all authenticated users using
`loginctl list-user` and runs `daemon-reload` for each of them.

This is a first step towards a `nixos-rebuild` which is able to reload
user units from systemd. The entire task is fairly hard, however I
consider this patch usable as it allows to restart units without running
`daemon-reload` for each authenticated user.

nixos/doc/manual/release-notes/rl-1809.xml

···
       The module option <option>nix.useSandbox</option> is now defaulted to <literal>true</literal>.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      The config activation script of <literal>nixos-rebuild</literal> now
+      <link xlink:href="https://www.freedesktop.org/software/systemd/man/systemctl.html#Manager%20Lifecycle%20Commands">reloads</link>
+      all user units for each authenticated user.
+    </para>
+  </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/system/activation/switch-to-configuration.pl

···
 # Make systemd reload its units.
 system("@systemd@/bin/systemctl", "daemon-reload") == 0 or $res = 3;
 
+# Reload user units
+open my $listActiveUsers, '-|', '@systemd@/bin/loginctl', 'list-users', '--no-legend';
+while (my $f = <$listActiveUsers>) {
+    next unless $f =~ /^\s*(?<uid>\d+)\s+(?<user>\S+)/;
+    my ($uid, $name) = ($+{uid}, $+{user});
+    print STDERR "reloading user units for $name...\n";
+
+    system("su", "-l", $name, "-c", "XDG_RUNTIME_DIR=/run/user/$uid @systemd@/bin/systemctl --user daemon-reload");
+}
+
+close $listActiveUsers;
+
 # Set the new tmpfiles
 print STDERR "setting up tmpfiles\n";
 system("@systemd@/bin/systemd-tmpfiles", "--create", "--remove", "--exclude-prefix=/dev") == 0 or $res = 3;