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-cleanup.c

@@ -308,6 +308,15 @@ cleanup_old_deployments (OstreeSysroot       *self,
       g_hash_table_replace (active_boot_checksums, bootcsum, bootcsum);
     }
 
+  /* And also the staged deployment, if any */
+  if (self->staged_deployment)
+    {
+      char *deployment_path = ostree_sysroot_get_deployment_dirpath (self, self->staged_deployment);
+      g_hash_table_replace (active_deployment_dirs, deployment_path, deployment_path);
+      char *bootcsum = g_strdup (ostree_deployment_get_bootcsum (self->staged_deployment));
+      g_hash_table_replace (active_boot_checksums, bootcsum, bootcsum);
+    }
+
   /* Find all deployment directories, both active and inactive */
   g_autoptr(GPtrArray) all_deployment_dirs = NULL;
   if (!list_all_deployment_directories (self, &all_deployment_dirs,