libstage: add stage, rename adopt->replace in stage_conf_t

In preparation for supporting caller-supplied stage memory, add another
stage_t pointer to the conf called .stage.

The naming of .adopt was always awkward and resulted from the reuse of
the .parent pointer to indicate the stage to replace.  Now that there's
a .stage member, just rename .adopt to .replace, with the same semantics.

At some point I need to firm up what .replace does with the other conf
data.  Currently, only the name, ops, and object, are updated.  The
existing active/locked/alpha etc are retained unchanged, ignored from
the conf.  This can be surprising/unexpected to someone performing a
replace through the .conf, and it would probably be convenient to be
able to set everything during a replace.  So there should probably be
a convenience helper for constructing a conf from an existing stage,
which can then be overridden selectively while passing to a new().

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