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-private.h

@@ -61,6 +61,8 @@ struct OstreeSysroot {
   int bootversion;
   int subbootversion;
   OstreeDeployment *booted_deployment;
+  OstreeDeployment *staged_deployment;
+  GVariant *staged_deployment_data;
   struct timespec loaded_ts;
 
   /* Only access through ostree_sysroot_[_get]repo() */
@@ -71,6 +73,7 @@ struct OstreeSysroot {
 
 #define OSTREE_SYSROOT_LOCKFILE "ostree/lock"
 /* We keep some transient state in /run */
+#define _OSTREE_SYSROOT_RUNSTATE_STAGED "/run/ostree/staged-deployment"
 #define _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_DIR "/run/ostree/deployment-state/"
 #define _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_FLAG_DEVELOPMENT "unlocked-development"
 
@@ -105,6 +108,22 @@ _ostree_sysroot_list_deployment_dirs_for_os (int                  deploydir_dfd,
                                              GCancellable        *cancellable,
                                              GError             **error);
 
+void
+_ostree_deployment_set_bootconfig_from_kargs (OstreeDeployment *deployment,
+                                              char            **override_kernel_argv);
+
+gboolean
+_ostree_sysroot_reload_staged (OstreeSysroot *self, GError       **error);
+
+gboolean
+_ostree_sysroot_finalize_staged (OstreeSysroot *self,
+                                 GCancellable  *cancellable,
+                                 GError       **error);
+
+OstreeDeployment *
+_ostree_sysroot_deserialize_deployment_from_variant (GVariant *v,
+                                                     GError  **error);
+
 char *
 _ostree_sysroot_get_origin_relpath (GFile         *path,
                                     guint32       *out_device,
@@ -118,6 +137,8 @@ _ostree_sysroot_rmrf_deployment (OstreeSysroot *sysroot,
                                  GCancellable  *cancellable,
                                  GError       **error);
 
+char * _ostree_sysroot_get_runstate_path (OstreeDeployment *deployment, const char *key);
+
 char *_ostree_sysroot_join_lines (GPtrArray  *lines);
 
 gboolean _ostree_sysroot_query_bootloader (OstreeSysroot     *sysroot,