RX: Avoid timing out non-kernel busy channels

RX: Avoid timing out non-kernel busy channels

When we encounter a "busy" call channel (indicated by receiving
RX_PACKET_TYPE_BUSY packets), we can error out a call with
RX_CALL_TIMEOUT to try and get the application code to retry the call.
However, many RX applications are not aware of this, and will just
fail with an error upon receiving a single busy packet.

So instead, make this behavior optional, and only do it if the
application tells us what specific error it expects to receive when a
busy call channel is detected. Enable this behavior for the Unix cache
manager, as it can cope with receiving an RX_CALL_TIMEOUT error in
this scenario.

Change-Id: I2fe03c0ff81576da3b2ce2b4d27a16d5631c073e
Reviewed-on: http://gerrit.openafs.org/4159
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>