Add the ipc shutdown event (#2652)

This event is triggered when the connection to the ipc is about to
shutdown because of a user action such as with a `restart` or `exit`
command. The `change` field indicates why the ipc is shutting down. It
can be either "restart" or "exit".

fixes #2318

include/ipc.h

@@ -77,11 +77,18 @@ int ipc_create_socket(const char *filename);
 void ipc_send_event(const char *event, uint32_t message_type, const char *payload);
 
 /**
- * Calls shutdown() on each socket and closes it. This function to be called
- * when exiting or restarting only!
+ * Calls to ipc_shutdown() should provide a reason for the shutdown.
+ */
+typedef enum {
+    SHUTDOWN_REASON_RESTART,
+    SHUTDOWN_REASON_EXIT
+} shutdown_reason_t;
+
+/**
+ * Calls shutdown() on each socket and closes it.
  *
  */
-void ipc_shutdown(void);
+void ipc_shutdown(shutdown_reason_t reason);
 
 void dump_node(yajl_gen gen, Con *con, bool inplace_restart);