Fixed dirpath generation on OSX with Filesystem driver

Since 43729fb, Filesystem driver checks path length for Windows (limited to 260 chars by Microsoft).
However, Windows check is made with `stristr(PHP_OS,'WIN')`, which makes it search for `WIN` string in all `PHP_OS`. This makes it fail on OSX, where `PHP_OS` == `Darwin`.

This patch checks if `WIN` is at the beginning of `PHP_OS` string. According to [PHP doc](http://fr2.php.net/php_uname), value on Windows can be `WIN32`, `WINNT` or `Windows`.