Return the correct status when a partially completed request is cancelled.

Return the correct status when a partially completed request is cancelled.

After the previous changes to fix requests on blocking sockets to complete
across multiple operations, an edge case exists where a request can be
cancelled after it has partially completed.  POSIX doesn't appear to
dictate exactly how to handle this case, but in general I feel that
aio_cancel() should arrange to cancel any request it can, but that any
partially completed requests should return a partial completion rather
than ECANCELED.  To that end, fix the socket AIO cancellation routine to
return a short read/write if a partially completed request is cancelled
rather than ECANCELED.

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