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/libostree/ostree-sysroot.h

@@ -74,6 +74,8 @@ _OSTREE_PUBLIC
 GPtrArray *ostree_sysroot_get_deployments (OstreeSysroot  *self);
 _OSTREE_PUBLIC
 OstreeDeployment *ostree_sysroot_get_booted_deployment (OstreeSysroot *self);
+_OSTREE_PUBLIC
+OstreeDeployment *ostree_sysroot_get_staged_deployment (OstreeSysroot *self);
 
 _OSTREE_PUBLIC
 GFile *ostree_sysroot_get_deployment_directory (OstreeSysroot    *self,
@@ -174,6 +176,17 @@ gboolean ostree_sysroot_deploy_tree (OstreeSysroot     *self,
                                      GCancellable      *cancellable,
                                      GError           **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_sysroot_stage_tree (OstreeSysroot     *self,
+                                    const char        *osname,
+                                    const char        *revision,
+                                    GKeyFile          *origin,
+                                    OstreeDeployment  *merge_deployment,
+                                    char             **override_kernel_argv,
+                                    OstreeDeployment **out_new_deployment,
+                                    GCancellable      *cancellable,
+                                    GError           **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_sysroot_deployment_set_mutable (OstreeSysroot     *self,
                                                 OstreeDeployment  *deployment,