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

apidoc/ostree-sections.txt

@@ -170,6 +170,7 @@ ostree_deployment_get_origin
 ostree_deployment_get_origin_relpath
 ostree_deployment_get_unlocked
 ostree_deployment_is_pinned
+ostree_deployment_is_staged
 ostree_deployment_set_index
 ostree_deployment_set_bootserial
 ostree_deployment_set_bootconfig
@@ -506,6 +507,7 @@ ostree_sysroot_cleanup
 ostree_sysroot_prepare_cleanup
 ostree_sysroot_repo
 ostree_sysroot_get_repo
+ostree_sysroot_get_staged_deployment
 ostree_sysroot_init_osname
 ostree_sysroot_deployment_set_kargs
 ostree_sysroot_deployment_set_mutable
@@ -514,6 +516,7 @@ ostree_sysroot_deployment_set_pinned
 ostree_sysroot_write_deployments
 ostree_sysroot_write_deployments_with_options
 ostree_sysroot_write_origin_file
+ostree_sysroot_stage_tree
 ostree_sysroot_deploy_tree
 ostree_sysroot_get_merge_deployment
 ostree_sysroot_query_deployments_for