Add placeholder functions for "repmgr $command --help"

There are now too many options to sensibly fit into general --help
output; we'll add separate output for each repmgr command, e.g.
"repmgr node --help".

repmgr-action-standby.c

@@ -4065,3 +4065,12 @@ parse_node_check_replication_lag(const char *node_check_output, int *seconds, in
 
 	return status;
 }
+
+
+void
+do_standby_help(void)
+{
+	print_help_header();
+
+
+}