[core] Add SHUTDOWN_REQUESTED event. Bind to this event to know when ASAP or elegant shutdown has been requested.

src/include/switch_types.h

@@ -2048,6 +2048,7 @@ typedef uint32_t switch_io_flag_t;
     SWITCH_EVENT_CALL_SETUP_RESULT
     SWITCH_EVENT_CALL_DETAIL
     SWITCH_EVENT_DEVICE_STATE
+    SWITCH_EVENT_SHUTDOWN_REQUESTED		- Shutdown of the system has been requested
     SWITCH_EVENT_ALL				- All events at once
 </pre>
 
@@ -2143,6 +2144,7 @@ typedef enum {
 	SWITCH_EVENT_CALL_DETAIL,
 	SWITCH_EVENT_DEVICE_STATE,
 	SWITCH_EVENT_TEXT,
+	SWITCH_EVENT_SHUTDOWN_REQUESTED,
 	SWITCH_EVENT_ALL
 } switch_event_types_t;