1) If we get an unexpected disconnect, issue a REQUEST SENSE, as recommended
by the SCSI-2 spec. If the target created a contingent allegiance condition,
this will clear it. Also, if it happened while sending a SDTR or WDTR message,
disable negotiation for that target.
2) Since some lame devices still don't deal correctly, make sure we deassert
ATN if our last message out is interrupted. If we get a MESSAGE PARITY ERROR,
we'll reassert ATN anyway. This should ensure that we never have to send a
MESSAGE NO OPERATION.
3) Set AIC_ABORTING only when actually sending a BUS DEVICE RESET or ABORT,
so we get better error detection.
4) Other internal reorganization of no consequence.
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