Restore "admin dir" warning on login page

Commit 9da643a modified the admin
checks on login page to remove the logic checking for pre 1.0 upgrade
steps.

However, it also (probably unintentionally) removed the check for admin
directory presence, so administrators are no longer reminded that they
should delete this directory, potentially leaving them exposed to
security breaches.

This commit restores the warning, and improves the error message.

Fixes #23179
Stopgap measure for issue #23173

lang/strings_english.txt

@@ -656,7 +656,7 @@ $s_click_to_login = 'Login';
 # login.php
 $s_warning_plain_password_authentication = '<strong>Warning:</strong> Plain password authentication is used, this will expose your passwords to administrators.';
 $s_warning_default_administrator_account_present = '<strong>Warning:</strong> You should disable the default \'administrator\' account or change its password.';
-$s_warning_admin_directory_present = '<strong>Warning:</strong> Admin directory should be removed.';
+$s_warning_admin_directory_present = '<strong>Warning:</strong> "admin" directory should be removed, or access to it restricted.';
 $s_warning_change_setting = '<strong>Warning:</strong> "%1$s" is not set to its default value (%2$s).';
 $s_warning_security_hazard = 'This is a potential security hazard as it can expose sensitive information.';
 $s_warning_integrity_hazard = 'This will cause MantisBT to continue when errors occurs and may lead to system/data integrity issues.';

login_page.php

@@ -140,6 +140,13 @@
 $t_warnings = array();
 $t_upgrade_required = false;
 if( config_get_global( 'admin_checks' ) == ON && file_exists( dirname( __FILE__ ) .'/admin' ) ) {
+	# Check if the admin directory is accessible
+	$t_admin_dir = dirname( __FILE__ ) . '/admin';
+	$t_admin_dir_is_accessible = @file_exists( $t_admin_dir . '/.' );
+	if( $t_admin_dir_is_accessible ) {
+		$t_warnings[] = lang_get( 'warning_admin_directory_present' );
+	}
+
 	# Generate a warning if default user administrator/root is valid.
 	$t_admin_user_id = user_get_id_by_name( 'administrator' );
 	if( $t_admin_user_id !== false ) {