Allows do-install.php only if data/do-install.txt exists

https://github.com/FreshRSS/FreshRSS/issues/1449

cli/do-install.php

@@ -3,6 +3,10 @@
 require('_cli.php');
 require(LIB_PATH . '/lib_install.php');
 
+if (!file_exists(DATA_PATH . '/do-install.txt')) {
+	fail('FreshRSS looks to be already installed! Please use `./cli/reconfigure.php` instead.');
+}
+
 $params = array(
 		'environment:',
 		'base_url:',