tmpfiles: automatically remove old machine snapshots at boot

Remove old temporary snapshots, but only at boot. Ideally we'd have
"self-destroying" btrfs snapshots that go away if the last last
reference to it does. To mimic a scheme like this at least remove the
old snapshots on fresh boots, where we know they cannot be referenced
anymore. Note that we actually remove all temporary files in
/var/lib/machines/ at boot, which should be safe since the directory has
defined semantics. In the root directory (where systemd-nspawn
--ephemeral places snapshots) we are more strict, to avoid removing
unrelated temporary files.

This also splits out nspawn/container related tmpfiles bits into a new
tmpfiles snippet to systemd-nspawn.conf

src/nspawn/nspawn.c

@@ -4522,9 +4522,9 @@ int main(int argc, char *argv[]) {
                                 goto finish;
                         }
                         if (r > 0)
-                                r = tempfn_random_child(arg_directory, NULL, &np);
+                                r = tempfn_random_child(arg_directory, "machine.", &np);
                         else
-                                r = tempfn_random(arg_directory, NULL, &np);
+                                r = tempfn_random(arg_directory, "machine.", &np);
                         if (r < 0) {
                                 log_error_errno(r, "Failed to generate name for snapshot: %m");
                                 goto finish;