cmd_stat: ack the user when turning off stat reporting

If we are master (or slave with no master running) the next commit
makes stats completely silent (nothing happens: nothing printed).
A user typing "stat" won't know if it turned them on or off. So
turning on is now printed:

   wrc# stat
   wrc#
   wrc#
   wrc# stat
   statistics now off

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>