Remove a couple of disused structure elements.
Reinstate my changes to the message-handling code, ensuring that we don't
release ACK until after we've decided whether we need to reply to a message,
since if we reply, we have to assert ATN before releasing ACK.
Rather than doing lots of sums every time we need to convert between
SCSI transfer period factors and WD33C93 TPx bits, build a little lookup
table of the available TPFs at startup and use that.  This is rather
easier to understand than the old way.

Connectedly, remove code that tested for sc_minsync==0, since that couldn't
reasonably be true.  Clients can use cf_flags to disable sync negotiation
if necessary.
Ignoring WDTR IN is rude.  Respond instead with MESSAGE REJECT OUT, like
any good initiator should.
Print the attachment message in the attach routine rather than in the bus
reset routine.  This should avoid getting odd messages when things go badly
wrong.  While I'm there, re-arrange the attach message a bit (to look more
like the NCR53C9x one) and add the clock frequency to it.
Add a couple of debugging messages for tracing transfer mode discussions
with the scsipi layer.
At the end of sync transfer negotiations, record the actual synchronous
transfer agreement that's been reached, rather than rounding it up to
the next transfer period we can support.  This is particularly important
for target-initiated negotiations, since otherwise it might end up
transmitting at a lower than necessary rate.

Also, if we've agreed on sync transfer, don't silently try to use async
transfers just because the transfer period we've negotiated is a bit long.
It's possible we should renegotiate async transfers at that point, but for
now we'll just put up with it.
Add support for negotiating async transfers if the scsipi layer asks for
that, or if the target requests sync transfers but the scsipi layer hasn't
turned them on yet.  This is still some way from correctly implementing
the rules in SPI-5, but it's closer than it was.
When reselecting a transfer that had DMA disabled, keep DMA disabled.
Remove a rogue printf that crept into the last commit.
Pull up timecounter fix from trunk.
Add a definition of T_WANTSYNC, which I somehow missed committing.