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-deployment.c

@@ -339,3 +339,16 @@ ostree_deployment_is_pinned (OstreeDeployment *self)
     return FALSE;
   return g_key_file_get_boolean (self->origin, OSTREE_ORIGIN_TRANSIENT_GROUP, "pinned", NULL);
 }
+
+/**
+ * ostree_deployment_is_staged:
+ * @self: Deployment
+ *
+ * Returns: `TRUE` if deployment should be "finalized" at shutdown time
+ * Since: 2018.3
+ */
+gboolean
+ostree_deployment_is_staged (OstreeDeployment *self)
+{
+  return self->staged;
+}