Add concept of "staged" deployment

Add API to write a deployment state to `/run/ostree/staged-deployment`,
along with a systemd service which runs at shutdown time.

This is a big change to the ostree model for hosts,
but it closes a longstanding set of bugs; many, many people have
hit the "losing changes in /etc" problem.  It also avoids
the other problem of racing with programs that modify `/etc`
such as LVM backups:
https://bugzilla.redhat.com/show_bug.cgi?id=1365297

We need this in particular to go to a full-on model for
automatically updated host systems where (like a dual-partition model)
everything is fully prepared and the reboot can be taken
asynchronously.

Closes: https://github.com/ostreedev/ostree/issues/545

Closes: #1503
Approved by: jlebon

src/boot/ostree-finalize-staged.service

@@ -0,0 +1,36 @@
+# Copyright (C) 2018 Red Hat, Inc.
+#
+# This library 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 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the
+# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+# Boston, MA 02111-1307, USA.
+
+# For some implementation discussion, see:
+# https://lists.freedesktop.org/archives/systemd-devel/2018-March/040557.html
+[Unit]
+Description=OSTree Finalize Staged Deployment
+ConditionPathExists=/run/ostree-booted
+DefaultDependencies=no
+
+RequiresMountsFor=/sysroot
+After=basic.target
+Before=multi-user.target final.target
+Conflicts=final.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStop=/usr/bin/ostree admin finalize-staged
+
+[Install]
+WantedBy=multi-user.target