0.9.3.15: debugger streams

  * flush all standard streams before prompting in the REPL and the
    debugger
  * prompt for restart goes to *debug-io*
  * disable-debugger, enable-debugger don't set *debug-io* anymore
  * run *invoke-debugger-hook* before *debugger-hook* => if the debugger
    is disabled then *debugger-hook* is not run
  * eof selects abort in the debugger