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-cmdprivate.h

@@ -34,6 +34,7 @@ typedef struct {
   gboolean (* ostree_static_delta_query_exists) (OstreeRepo *repo, const char *delta_id, gboolean *out_exists, GCancellable *cancellable, GError **error);
   gboolean (* ostree_static_delta_delete) (OstreeRepo *repo, const char *delta_id, GCancellable *cancellable, GError **error);
   gboolean (* ostree_repo_verify_bindings) (const char *collection_id, const char *ref_name, GVariant *commit, GError **error);
+  gboolean (* ostree_finalize_staged) (OstreeSysroot *sysroot, GCancellable *cancellable, GError **error);
 } OstreeCmdPrivateVTable;
 
 /* Note this not really "public", we just export the symbol, but not the header */