Tell a refused inline apart from an absent owner

[SimonCropp]

TrySendAsync collapsed an error reply and a refused connection into the same
false, and AddInlineAsync read that as "nobody owns the queue" and launched a
viewer. But an owner that answered is still there and still holds the port, so
the launched viewer cannot bind it - and AddInlineAsync returned Queued anyway,
as soon as stdin was written.

So a snapshot the owner declined - an older owner that does not understand the
payload, or a handler that threw - was reported to the caller as queued while
being held by nothing at all. Nothing was staged either, because staging is what
NoViewerFound is for. The snapshot simply vanished, and the test that produced
it went on reporting a failure with nowhere to review it.

SendAsync returns three outcomes instead of two: NoOwner, Accepted, Refused.
TrySendAsync stays as it was, as a wrapper, so PendingFiles and the rest are
untouched. AddInlineAsync launches a viewer only on NoOwner, and reports
NoViewerFound on a refusal - which is accurate from the caller's side, since in
both cases the snapshot is pending nowhere and staging is the right answer.

The rest of that item - RunInline ignoring response.Ok, and persisting through
InlineStaging on a forward failure - is in DiffEngineViewer and not touched
here.