Handle the pt-spec termination requirements (Fixes #12)

 * Does the right thing according to the tor pt-spec when SIGINTs are received.
 * As part of debugging this, there's a new (undocumented) command line option
   --wait-for-debugger, that sleeps in a tight loop early in the app execution.
   Attach the debugger and set wait_for_debugger to false for fun times.