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.h

@@ -73,7 +73,8 @@ OstreeBootconfigParser *ostree_deployment_get_bootconfig (OstreeDeployment *self
 _OSTREE_PUBLIC
 GKeyFile *ostree_deployment_get_origin (OstreeDeployment *self);
 
-
+_OSTREE_PUBLIC
+gboolean ostree_deployment_is_staged (OstreeDeployment *self);
 _OSTREE_PUBLIC
 gboolean ostree_deployment_is_pinned (OstreeDeployment *self);