This patch adds a flag that requests the target agent to make the trace 
buffer circular, so that instead of filling it up and then stopping, the 
agent discards the oldest trace frames as necessary to accommodate new 
ones.  Any hairy memory management code is going to be on the target 
side; GDB just has to transmit the setting (and now always via target 
vector), and report back status, which may now include a total number of 
frames that were created.  This also adds complete documentation of the 
qTStatus reply, per request.  Any comments before I commit?

Stan

2010-03-16 Stan Shebs  <stan@codesourcery.com>
           Pedro Alves  <pedro@codesourcery.com>

        * target.h (struct target_ops): New method
        to_set_circular_trace_buffer.
        (target_set_circular_trace_buffer): New macro.
        * target.c (update_current_target): Add
        to_set_circular_trace_buffer, fix to_set_disconnected_tracing
        default behavior.
        * remote.c (remote_set_circular_trace_buffer): New function.
        (init_remote_ops): Add it to vector.
        * tracepoint.h (struct trace_status): New field traceframes_created,
        change buffer_size and buffer_free to int.
        * tracepoint.c (circular_trace_buffer): New global.
        (start_tracing): Send values of disconnected tracing and circular
        trace buffer settings.
        (set_circular_trace_buffer): New function.
        (parse_trace_state): Handle total space and frames created.
        (trace_status_command): Display total space and total frames
        created.
        (trace_save): Write out new status values.
        (parse_trace_status): Set traceframe_count, traceframes_created,
        buffer_free and buffer_size to -1 by default.
        (_initialize_tracepoint): New setshow for circular-trace-buffer.
        * NEWS: Mention the circular trace buffer option.

        [gdb/doc]
        * gdb.texinfo (Starting and Stopping Trace Experiments): Describe
        circular-trace-buffer.
        (Tracepoint Packets): Describe QTBuffer, and details of the
        qTStatus reply.