[MEDIUM] signals: support redistribution of signal zero when stopping

[MEDIUM] signals: support redistribution of signal zero when stopping

Signal zero is never delivered by the system. However having a signal to
which functions and tasks can subscribe to be notified of a stopping event
is useful. So this patch does two things :
  1) allow signal zero to be delivered from any function of signal handler
  2) make soft_stop() deliver this signal so that tasks can be notified of
     a stopping condition.