Ironically, after all that, I ended up going with Pedro's pair of cancel 
queries - there was no way to do nested query() calls so that answering 
two questions would be the exception rather than the rule, and a 
three-answer query seemed like it was going to be a mouthful to 
explain.  So here's what I ended up committing.

Stan

2010-04-08  Stan Shebs  <stan@codesourcery.com>
        Pedro Alves  <pedro@codesourcery.com>

    * tracepoint.h (struct trace_status): New fields disconnected_tracing
    and circular_buffer.
    (disconnect_tracing): Rename from disconnect_or_stop_tracing.
    * tracepoint.c (trace_status_command): Display target's status for
    disconnected tracing and circular buffer.
    (disconnect_tracing): Rename from disconnect_or_stop_tracing, add
    query for non-disconnected-tracing case, remove the stop_tracing
    call.
    (tfile_open): Clear disconnected and circular buffer status.
    (trace_save): Save disconnected and circular buffer status.
    (parse_trace_status): Parse disconnected and circular buffer status,
    also recognize disconnected as a stop reason.
    * remote.c (remote_set_disconnected_tracing): Only set
    QTDisconnected if the remote end supports disconnected tracing.
    Warn otherwise, if trying to enable disconnected tracing.
    * infcmd.c (detach_command): Update disconnect_tracing call.
    * cli/cli-cmds.c (quit_command): Ditto.

    * gdb.texinfo (Tracepoint Packets): Describe disconn and circular
    trace status fields.