Implements undo command and optional timeout for configuration

Several new configure command variants:

configure undo - undo last reconfiguration
configure timeout - configure with scheduled undo if not confirmed in timeout
configure confirm - confirm last configuration
configure check - just parse and validate config file

nest/cli.h

@@ -49,6 +49,8 @@ typedef struct cli {
 extern pool *cli_pool;
 extern struct cli *this_cli;		/* Used during parsing */
 
+#define CLI_ASYNC_CODE 10000
+
 /* Functions to be called by command handlers */
 
 void cli_printf(cli *, int, char *, ...);