screen: write as a fresh actor when another node writes as this one

With --keep-actor, a node on a document file that ends with a clean stop
writes on as the actor the file names. Two nodes started on copies of
one file (a copied data directory, a restored disk image while the
original runs) both do, and both make that actor's next change number:
peers holding one refuse the other as forged, and everything built on
it.

screen::apply_bytes, the way every received change comes in, now looks
for a change of the actor the node writes as that the node doesn't hold:
only another node can have made it. The node then writes on as a fresh
actor at once, logs a warning, and its callers add the actor to the
node's own (RemakeState::add_own_actor), which the document file's
header follows. The change is judged as any other. lifecycle.md says not
to start a copy of a node's data directory with --keep-actor, and what
the switch leaves: what both wrote before stays refused where the
other's came first.

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