Corner cases in CSR handshake with host

1) The spec. says that 'all bits of BSEL2' are cleared at the completion
of an input command.  We weren't doing that.  In fact, we can't always,
since an input command might result in an immediate completion.

This may fix the report that "PDP11s hung at IPL5 when initializing the
device".

If not, I'd like an instruction trace.

2) If RQI is left asserted by the host during an input command, the KMC
is not supposed to output any completions; the host gets another input
command.  We *almost* guaranteed this before.  This closes an obscure
case where we didn't.