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-private.h

@@ -36,6 +36,7 @@ G_BEGIN_DECLS
  * @bootconfig: Bootloader configuration
  * @origin: How to construct an upgraded version of this tree
  * @unlocked: The unlocked state
+ * @staged: TRUE iff this deployment is staged
  */
 struct _OstreeDeployment
 {
@@ -50,6 +51,7 @@ struct _OstreeDeployment
   OstreeBootconfigParser *bootconfig;
   GKeyFile *origin;
   OstreeDeploymentUnlockedState unlocked;
+  gboolean staged;
 };
 
 void _ostree_deployment_set_bootcsum (OstreeDeployment *self, const char *bootcsum);