libstage: introduce 'adopt' flag to stage_conf_t

When set, this turns stage_new() into a stage_replace() against the
supplied parent.

So if you have an existing stage_t and you wish to turn it into
something else directly via stage_new(), simply pass it in as the
parent with the adopt flag set.

Previously it was rather contorted to do this kind of thing, and
kind of impossible really.  This way avoids the allocation of the
new stage_t while preserving the usability of all existing components
build around the stage_new/stage_conf_t API.

This is kind of experimental and I haven't tested it yet, just a wart
I tripped over in the "sars" Blender 2020 project and wanted to do
something quick about before it falls off my radar as I'm busy with
unrelated things.

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