lib/sysroot: Move staged into deployment list, rework handling

lib/sysroot: Move staged into deployment list, rework handling

Followup to: https://github.com/ostreedev/ostree/pull/1503
After starting some more work on on this in rpm-ostree, it is
actually simpler if the staged deployment just shows up in the list.

It's effectively opt-in today; down the line we may make it the default,
but I worry about breaking things that e.g. assume they can mutate
the deployment before rebooting and have `/etc` already merged.

There's not that many things in libostree that iterate over the deployment
list.  The biggest change here is around the
`ostree_sysroot_write_deployments_with_options` API.  I initially
tried hard to support a use case like "push a rollback" while retaining
the staged deployment, but everything gets very messy because that
function truly is operating on the bootloader list.

For now what I settled on is to just discard the staged deployment;
down the line we can enhance things.

Where we then have some new gymnastics is around implementing
the finalization; we need to go to some effort to pull the staged
deployment out of the list and mark it as unstaged, and then pass
it down to `write_deployments()`.

Closes: #1539
Approved by: jlebon