Add support for the CUDA_RESTART_SYSTEM command

There are cases where when it's necessary (e.g. given uninitialized NVRAM,
the Beige G3 with the 10.2 install CD inserted will update the boot
device and restart to boot from it).

Restart support was done by wrapping the ppc_exec function in a loop and
checking for a restart power off reason. We also need to disconnect all
event listeners, since they will be recreated when the machine is
re-initialized.

debugger/debugger.cpp

@@ -458,6 +458,10 @@ void enter_debugger() {
             power_off_reason = po_shutting_down;
             break;
         }
+        if (power_off_reason == po_restart) {
+            power_off_reason = po_restarting;
+            break;
+        }
         power_on = true;
 
         if (power_off_reason == po_starting_up) {