This patch simply requires that there be at least one enabled tracepoint 
before a trace run will start.  While it's not necessarily wrong to have 
a tracepoint-less trace run, this could save a user or two from massive 
puzzlement as to why there are no trace frames accumulating. :-)  While 
this seems uncontroversial, I'm going to hold off commit to allow comments.

Stan

2010-03-26  Stan Shebs  <stan@codesourcery.com>

    * tracepoint.c (start_tracing): Check tracepoints before sending
    commands to target, don't start if all tracepoints disabled.