Don't suggest chmod to 777

Signed-off-by: Edward Z. Yang <ezyang@meta.com>

library/HTMLPurifier/DefinitionCache/Serializer.php

@@ -287,13 +287,14 @@ class HTMLPurifier_DefinitionCache_Serializer extends HTMLPurifier_DefinitionCac
             } elseif (filegroup($dir) === posix_getgid()) {
                 $chmod = $chmod | 0070;
             } else {
-                // PHP's probably running as nobody, so we'll
-                // need to give global permissions
-                $chmod = $chmod | 0777;
+              // PHP's probably running as nobody, it is
+              // not obvious how to fix this (777 is probably
+              // bad if you are multi-user), let the user figure it out
+                $chmod = null;
             }
             trigger_error(
-                'Directory ' . $dir . ' not writable, ' .
-                'please chmod to ' . decoct($chmod),
+                'Directory ' . $dir . ' not writable. ' .
+                ($chmod === null ? '' : 'Please chmod to ' . decoct($chmod)),
                 E_USER_WARNING
             );
         } else {