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.c

@@ -26,7 +26,7 @@
 #include "ostree-core-private.h"
 #include "ostree-repo-pull-private.h"
 #include "ostree-repo-static-delta-private.h"
-#include "ostree-sysroot.h"
+#include "ostree-sysroot-private.h"
 #include "ostree-bootloader-grub2.h"
 
 #include "otutil.h"
@@ -52,7 +52,8 @@ ostree_cmd__private__ (void)
     _ostree_repo_static_delta_dump,
     _ostree_repo_static_delta_query_exists,
     _ostree_repo_static_delta_delete,
-    _ostree_repo_verify_bindings
+    _ostree_repo_verify_bindings,
+    _ostree_sysroot_finalize_staged,
   };
 
   return &table;