factory-reset: revamp infrastructure

This introduces a bunch of facilities:

1. The factory-reset.target unit that requests a factory reset is now
   complemented by factory-reset-now.target that executes it at next
   boot.

2. This latter is added to the initial transaction via the new trivial
   systemd-factory-reset-generator.

3. A tool systemd-factory-reset has been added to query, request,
   cancel, complete factory reset operations (via EFI variables). Two of
   these are wrapped into units that are plugged into
   factory-reset.target and factory-reset-now.target respectively. The
   tool also provides a simple Varlink API.

This should make things a lot cleaner, and both be useful as explicit
implementation on UEFI, and as template + hookpoints for alternative
implementations on non-UEFI.

units/systemd-factory-reset-reboot.service

@@ -0,0 +1,17 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Reboot to Execute Factory Reset
+Documentation=man:systemd.special(7)
+DefaultDependencies=no
+After=factory-reset.target
+Conflicts=shutdown.target
+Before=shutdown.target
+SuccessAction=reboot